Image projection system

ABSTRACT

An image projection system for generating an image on a projection screen using a highly compact geometry. The optical system uses polarized light manipulated by at least one of a conicoid, or plane optical elements to effect a folded mirror system to project an image onto the screen.

This application is a continuation of U.S. application Ser. No.08/724,734, filed Sep. 30, 1996, now U.S. Pat. No. 5,975,703.

As part of this specification a microfiche appendix has been preparedwith one page of fiche having a total 36 frames, including the testtarget frame.

The present invention is concerned generally with an optical system andmethod for generating an image on a projection screen using a highlycompact geometry. More particularly, the optical system uses polarizedlight manipulated by at least one of a conicoid, or plane opticalelements to effect a folded mirror system to project an image onto ascreen.

Currently available image projection systems are quite large with theirdimensions (particularly the cabinet depth) making such systemscumbersome and requiring special preparation of a space for their use.Furthermore, in such projection systems which employ LCDs the lightoutput from the source has all polarized states but the system makes useof only one state of polarization, thus eliminating about half the lightavailable for imaging on the projection screen.

It is, therefore, an object of the system to provide an improved imageprojection system and method of use.

It is another object of the system to provide a novel system and methodfor projecting an image on a screen using a highly compact opticalsystem.

It is a further object of the invention to provide an improved systemand method for processing polarized input light using plane reflectingand transmitting optical elements.

It is a further object of the invention to provide an improved systemand method for processing polarized input light using conicoidal opticalelements.

It is yet another object of the invention to provide an improved systemand method for manipulating polarized light using a primary paraboloidal(or modified paraboloidal) element which is coaxially aligned with aninner, smaller secondary hyperboloidal (or modified hyperboloidal)element or ellipsoidal (or modified ellipsoidal) element to output asingle polarization state image for display on a projection screen.

It is yet a further object of the invention to provide an improvedsystem and method for manipulating polarized light using a convexconicoidal reflecting surface, a negative lens, a polarization-selectiveand converting reflecting/transmitting plane and a Fresnel lens, so asto output a single polarization state image for display on a projectionscreen.

It is also an object of the invention to provide an improved system andmethod for manipulating polarized light using a convex conicoidalreflecting surface, a polarization converting plane, apolarization-selective mirror plane, a positive lens section and aFresnel lens, to output a single polarization state image for display ona projection screen.

It is yet another object of the invention to provide an improved systemand method for manipulating polarized light using a primary concaveconicoidal reflector which is coaxially aligned with an inner, smallersecondary convex conicoid reflector that converts polarization state andthat selectively reflects/transmits depending on polarization state tooutput a single polarization state image for display on a projectionscreen.

It is an additional object of the invention to provide a novel systemand method for supplying light components of substantially orthogonalpolarizations for separate areas of an image for output onto aprojection screen.

It is still another object of the invention to provide an improvedsystem and method for separating different light polarization states toreconstruct an image on a projection screen.

It is also an additional object of the invention to provide a novelsystem and method for providing light of a first polarization to a firstLCD region and light of another polarization to a second LCD region forcontrolled transmission of images onto a projection screen.

It is also an object of the invention to provide an improved method andsystem for providing light of different polarization states to an LCDwhich programmably transmits selected polarization states for imagedisplay on a projection screen.

It is also an additional object of the invention to provide a novelmethod and system including a voltage adjusted LCD for controlledtransmission of selected polarization states for reconstruction as animage on a projection screen.

It is yet a further object of the invention to provide a novel systemand method for splitting different light polarization states of an imageand using a compact mirror system to reassemble and display the imageonto a projection screen.

It is an additional object of the invention to provide an improvedsystem and method for manipulating polarized light using a polarizationconverting mirror plane that is optimally tilted with respect to areflecting plane whose reflectance or transmissivity depends onpolarization state and that is parallel to a viewing screen which canembody a Fresnel lens, to fit within the minimum possible volume and tooutput a single polarization state image for display on a projectionscreen.

It is another object of the invention to provide an improved method andsystem for controlling differently polarized light beams using a highlycompact planar mirror system in conjunction with polarization converterelements to output an image onto a projection screen.

It is another object of the invention to provide an improved system andmethod for controlling differently polarized light beams using ahighly-compact planar mirror system in conjunction with polarizationsplitting and converting elements to output an image onto a projectionscreen.

It is still another object of the invention to provide an improvedsystem and method using polarization splitter films to separatedifferent polarization states of an image for projection onto a screen.

It is another object of the invention to provide an improved opticalsystem and method for display of an image on a projection screen,including a highly compact lens and/or reflector system having a spatiallight modulator insensitive to polarization state of light.

It is also a further object of this invention to improve the contrast ofa projection screen system by placing the elements of a bracketing lenspair between the output of the illumination source and the entrancepupil of the projection lens.

It is still a further object of the invention to improve the throughputefficiency of a projection system by placing the positive and negativelens elements of an approximately telescopic lens pair between theillumination source output and the aperture of an SLM.

It is yet a further object of this invention to correct for aberrationsin isolated sections of a projection screen illumination system byincluding that section within the elements of a bracketing or otherspecified optical lens pair, using either conventional lens elements orlens elements with one or more of their surface functions modified withaspherizing terms.

It is yet a further object of the invention to provide a novel opticaldisplay system and method for generating tiled image portions which canbe assembled to produce an enlarged projection screen display of a fillcomposite image.

It is yet an additional object of the invention to provide a novelsystem and method for display of an image on a projection screen usingpolarized light and correcting for an image hole arising from a hole inthe light input structure of the system.

It is yet a further object of the invention to provide an improvedsystem and method for manipulating polarized light for display of animage on a projection screen using conicoidal elements coupled with abeam compressor element to eliminate an image hole arising from aphysical hole in one of the conicoidal elements.

It is still another object of the invention to provide improved methodsof expanding and compressing beams of light using physically separatedprismatic Fresnel-type layers or conic forms of refractive material.

It is also an additional object of the invention to provide a novelsystem and method for manipulating polarized light using at least oneogived or tilted conicoidal element to eliminate a hole in a displayimage arising from a physical hole in one of the conicoidal elements.

It is another object of the invention to provide an improved system andmethod for efficiently transforming the cross-sectional shape of anoptical system's light beam, from circular to rectangular, usingreciprocating conicoidal mirrors and a beam expander device to recyclelight from the periphery of the circular input beam, to the centralportion of the rectangular output beam, with good cross-sectional beamuniformity and without any light passing through or near the lightsource or arc.

It is still another object of the invention to provide an improvedsystem and method for efficiently transforming the cross-sectional shapeof an optical system's light beam, from circular to rectangular, usingan adiabatically varying lightpipe cross-sectional area combined with atotal internally reflecting non-imaging optic angle transformingelement.

It is another object of the invention to provide a compact means forconverting an unpolarized beam of rectangular cross-section into asingle rectangular beam divided into adjacent regions of uncontaminatedorthogonal polarizations, using combinations of prisms andpolarization-selective coatings.

It is still another object of the invention to provide a compact meansfor converting an unpolarized input beam into a polarized output beamfree of contaminating polarization states, using a conicoidalpolarization converting reflector with physical inlet hole combined withreciprocating composite lens elements and a flat or weakly curved planeof polarization selective material.

It is an additional object of the invention to provide an improvedsystem and method for manipulating unpolarized light by means ofreciprocating conicoidal mirrors, beam expanders, positive and negativelens elements and polarization-selective reflecting materials, so as tooutput a single beam of light having rectangular cross-section and twoadjacent regions of uncontaminated orthogonal polarizations.

It is a further object of the invention to provide an improved methodfor increasing the throughput efficiency function of an optical systemby means of a reverse raytrace process that interatively launches raysfrom the entrance pupil of a projection lens, back through designatedlaunch points on an SLM and through the system's interatively aspherizedlens and reflector surfaces, to a target area corresponding to thesystem's light source.

It is still a further object of the invention to provide an improvedmethod for increasing the throughput efficiency function of an opticalsystem by means of a reverse raytrace process that further includesweighting factors for the actual spatial and angular properties of thesystem's light source.

It is yet a further object of the invention to provide an improvedmethod for increasing the throughput efficiency function of an opticalsystem by means of a reverse raytrace process that further includesweighting factors for intrinsic brightness non-uniformities that areobserved on the system's projection screen or on the system's SLM(image) plane.

It is also an object of the invention to provide an improved system andmethod for producing and manipulating orthogonally polarized light ofselected colors using an LCD color-splitting prism cube,polarization-selective coatings and prism elements, so as to outputeither one tri-color beam composed of two uncontaminated orthogonalpolarization states, or two uncontaminated orthogonally polarizedtri-color beams, each having passed through separate portions of eachcolor's LCD image.

It is a further object of the invention to provide a novel opticalsystem using two cross-firing LCD color-splitting prism cubes andintervening polarization-selective coupling elements, for the purpose ofoutputting a single beam whose orthogonal polarization states correspondto separate color images, which then are processed for one ofthree-dimensional viewing, increased image resolution or imagecomparison.

It is also a further object of the invention to provide an improvedsystem and method having a folded mirror, asymmetrical arrangement witha polarization splitting (also referred equivalently as polarizationselective reflecting) mirror enabling substantial reduction of depth ofthe projection system.

Other objects and advantages of the invention will be apparent from thedetailed description and drawings described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a side view of a polarization-selective, split-imagefolded-optic rear-projection system with plane reflectors,

FIG. 1B is a front view of the system in FIG. 1A,

FIG. 1C is a top view of the system in FIG. 1A and

FIG. 1D is a schematic representation of the spatial light modulator,electronic driving circuitry for video images;

FIG. 2 shows a generalized image forming system with light source, SLM,projection lens and beam-splitter.

FIG. 3 illustrates a color image forming system with light source,polarization coupler, tri-color LCD filtering system, projection lensand beam-splitter.

FIG. 4A shows a conventional polarization conversion metal-retardationfilm bi-layer and

FIG. 4B shows further detail of the associated polarization conversionmechanism in FIG. 4A.

FIG. 5 illustrates the side sectional view of a prior art folded-opticrear-projection system;

FIG. 6 illustrates a front perspective view of the prior art system ofFIG. 5;

FIG. 7 illustrates a variation on the embodiment of FIG. 1A using acurved polarization-selective reflector;

FIG. 8 illustrates reflector shape differences between the embodimentsof FIG. 1A and FIG. 7;

FIG. 9 illustrates a variation on the embodiment of FIG. 1A using tiltedpolarization-converting mirrors and an alternative lens placement andalso shown is a magnified detail of an element of FIG. 9;

FIG. 10 illustrates a variation on the embodiment of FIG. 9, usingtilted polarization-converting mirrors and another alternative lensplacement;

FIG. 11 illustrates another form of the folded-optic rear-projectionsystem of FIG. 1A;

FIG. 12 illustrates another embodiment of the folded-opticrear-projection system of FIG. 11;

FIG. 13 illustrates a variation on the embodiment of FIG. 11 using acurved and tilted set of re-directing mirrors;

FIG. 14A shows a single-image beam variation on the embodiment of FIG.1A using polarization-selective and converting bi-layer with linearlypolarized input light and

FIG. 14B shows a single-image beam variation on the embodiment ofpolarization-selective and converting bi-layer of FIG. 1A withcircularly polarized input light;

FIG. 15 shows a form of the embodiment of FIG. 14A using tiltedpolarization-converting mirror plane and a vertical source-foldingmirror plane;

FIG. 16 illustrates a variation on the embodiments of FIG. 14 using acurved polarization-converting mirror also shown in magnified detail;

FIG. 17 illustrates a variation on FIG. 14A using left-handcircularly-polarized input light and a polarization-selective reflectordesigned for circular polarization;

FIG. 18 illustrates a variation on FIG. 14A using right-handcircularly-polarized input light and a polarization-selective reflectordesigned for circular polarization;

FIG. 19 illustrates a split-image variation on FIG. 14A using a verticalpolarization-converting mirror with axial light inlet hole;

FIG. 20 illustrates a variation on FIG. 19 using a curved polarizationconverting mirror with axial inlet hole;

FIG. 21 shows the side view of a tilt-angle variation of FIG. 19 toeliminate visual artifacts;

FIG. 22A shows a front view and

FIG. 22B a side view of a three-dimensionally shapedpolarization-converting mirror with ogive correction;

FIG. 23 shows hinged upper and lower polarization-converting mirrorplanes;

FIG. 24 shows the side view of an optical arrangement for eliminatingvisual artifacts caused by the inlet hole in embodiments of FIGS. 19-21;

FIG. 25A shows another system for eliminating visual artifacts using apolarization-selective window for an inlet hole and a reciprocatingmetal reflector and

FIG. 25B shows an alternative structure for the reciprocating outputreflector as a partial removed section;

FIG. 26 shows a generalized beam-displacement method for hiding a metalreflector;

FIG. 27 shows a prismatic beam-displacement arrangement for hiding ametal reflector;

FIG. 28 shows a perspective illustration of a prismatic beam-displacerelement;

FIG. 29 shows a ray-path sequence of a folded-optic mirror systems suchas in FIG. 1A;

FIG. 30 shows another ray path sequence as in FIG. 29 for systems of thetype shown in FIGS. 22-24;

FIG. 31 shows another ray path sequence as in FIG. 29 for systems of thetype shown in FIG. 14;

FIG. 32 shows a cross sectional view of a conicoidal variation on theembodiment of FIG. 19;

FIG. 33 is a three-dimensional perspective front view of the system ofFIG. 32;

FIG. 34 is a three dimensional perspective front view of the system ofFIG. 32 truncated for rectangular viewing;

FIG. 35 Illustrates a variation on the embodiment of FIG. 32;

FIG. 36 illustrates a variation of the embodiment of FIG. 32 using beamdisplacement elements and hole-elimination features;

FIG. 37 illustrates a variation of the embodiment of FIG. 35 arrangedfor diverging output light and Fresnel lens correction;

FIG. 38 illustrates a magnified view of the cross-sectional behavior ofthe embodiment of FIG. 37 showing its hole-eliminating features;

FIG. 39 illustrates the conic origin of conicoidal forms;

FIG. 40 illustrates a perspective view of optical behavior of a 3M-typelinear polarization-selective reflector film layer;

FIG. 41 shows a perspective view of the ray alignment implications ofFIG. 40 with preferred polarization orientations mapped onto a curvedsurface;

FIG. 42 shows a partial cross-sectional view of FIG. 41 ray alignmentwith curved reflector surface;

FIG. 43 shows various ray-film alignment situations for FIG. 41: i.parallel, ii. orthogonal and iii. oblique;

FIG. 44 shows reflected and transmitted ray splittings for obliquelyincident ray of polarization orthogonal to film of FIG. 40;

FIG. 45 shows experimentally determined reflectance and transmissiondata as a function of ray-film alignment angle for 0 and 45 degreeangles of incidence;

FIG. 46 shows the placement of pre-cut preferred-orientation film ringson a circumferentially-faceted secondary conicoid;

FIG. 47 shows the method of pre-cutting circumferential ring-sections ofthe film used in FIG. 46;

FIG. 48 shows a radially-faceted variation on FIG. 46;

FIG. 49 shows the method of pre-cutting radial facet-sections of thefilm used in FIG. 48;

FIG. 50 shows a cross sectional view of a variation on the embodimentsof FIGS. 32-38 using refractive elements polarization converting andselecting layers arranged as plane surfaces and also shown in phantom isan alternative portion for converting and selecting polarization;

FIG. 51 shows another form of the embodiment of FIG. 50 using a curvedreflector, composite positive and negative lens with flat polarizationconverting and selective reflecting plane;

FIG. 52 shows another embodiment as in FIG. 50 using a curved reflector,flat polarization converting and selective reflecting plane withtruncated plano-convex lens element;

FIG. 53 shows a variation on the embodiment of FIGS. 51 and 52;

FIG. 54 shows an example form of the embodiment of FIG. 51;

FIG. 55 shows another example form of the embodiment of FIG. 52;

FIG. 56 shows a polarization filtration element for split-imageprojection system with split polarizer and continuous substrate;

FIG. 57 shows a system like FIG. 56 but with split converting film andcontinuous polarizer;

FIG. 58 illustrates a conventional LCD structure cross-section;

FIG. 59 illustrates split-image form of FIG. 58 with split inputpolarizer and split alignment layer;

FIG. 60 shows another form of FIG. 59 with split input and outputpolarizers;

FIG. 61 shows a cross-sectional view of the pre-polarization ofunpolarized input light for a split-image LCD with a buffer zone;

FIG. 62 shows the cross-sectional view of orthogonally-polarized inputlight used with a split-image LCD with buffer zone;

FIG. 63 shows a perspective view of the spatial overlap between acircular input beam and the rectangular aperture of the split image LCDsystems of FIGS. 61-62;

FIG. 64 shows a perspective view of the spatial overlap between therectangular illumination beam and rectangular split-image LCD;

FIG. 65 shows a perspective view of a split-image LCD's rectangularoutput beam and polarization-sensitive beam-splitting;

FIG. 66 shows electronic programming of an image data stream with LCD(and other SLMs);

FIG. 67 shows the mechanism and corrections of keystone imagedistortions;

FIG. 68 shows the appearance of keystone distortion;

FIG. 69 shows electronic correction for keystone distortion;

FIG. 70 shows an image tilt method of distortion correction;

FIG. 71 shows image tilt path length correction with a refractive wedge;

FIG. 72 shows perspective relationships of keystone-distorted projectionsystem with optical path length correction;

FIG. 73 shows perspective relationships of electronically-correctedkeystone distortion in the projection system of FIG. 72;

FIG. 74 shows a polarization beam-splitter for pre-polarized lightincluding a director for split-image folded-optic projection systems;

FIG. 75 shows a polarization beam-splitter including beam directorarchitecture for unpolarized light;

FIG. 76 shows a prior art splitter;

FIG. 77 shows a prior art splitter;

FIG. 78 shows another prior art splitter;

FIG. 79 shows a split-image prism beam-splitter embodiment corrected foruse with light after a projection lens;

FIG. 80 shows optical beam size and path length relationships inprismatic beam-splitters;

FIG. 81 shows another split-image corrected prism embodiment for usewith light after a projection lens;

FIG. 82 shows a variation of a beam splitter embodiment with prismaticfilm beam directors;

FIG. 83 shows a negative lens variation of beam splitter embodiment foruse with converging input light;

FIG. 84 illustrates optical path length relationships in a projectionsystem;

FIG. 85 illustrates the use of a refractive element as an optical pathlength correction means in a projection system;

FIG. 86 illustrates a prior art reciprocating mirror method forillumination beam shape transformation;

FIG. 87 illustrates another prior art mirror system for beam shapetransformation;

FIG. 88 is a prior art paraboloidal (collimating) light source;

FIG. 89A is a perspective illustration of a conventional arc lamp and

FIG. 89B is a perspective display of the near-field brightnessdistribution of a conventional (d.c.) arc source;

FIG. 90A is a cross-sectional view of a beam shape embodiment withreciprocating mirrors arrangement within a converging light source andbeam-expander,

FIG. 90B is a cross-section of a beam profile along the line B—B in FIG.90A,

FIG. 90C is a view along line C—C toward the arc source of FIG. 90A, and

FIG. 90D is an alternative convex mirror for the embodiment of FIG. 90A;

FIG. 91A is a variation on the embodiment of FIG. 90 with a beamexpander and bracketing lens elements;

FIG. 91B shows a cross-sectional view along line B—B toward the arcsource in FIG. 91A; and

FIG. 91C is a magnified view of an alternative convex mirror for theembodiment of FIG. 91B;

FIG. 92 is of a conventional ellipsoidal (converging) light source;

FIG. 93A is a cross-sectional view of a variation of the embodiment ofFIG. 90 with a collimated light source and

FIG. 93B is an alternative convex mirror for the embodiment of FIG. 93A;

FIG. 94A is a cross-sectional view of a variation on the embodiment ofFIG. 90 using a collimating light and alternative mirror design,

FIG. 94B is a perspective view of one type of output mirror withrectangularly-shaped open-aperture used in FIG. 94A,

FIG. 94C is a magnified cross-sectional view of the reciprocatingmirrors of FIG. 94B,

FIG. 94D is a magnified cross-section of the small mirror in FIG. 94C,and

FIG. 94E shows a front sectional view of the beam profile taken alongline C—C in FIG. 94A;

FIG. 95 is a variation on the embodiment of FIG. 90 with a collimatedlight source, beam expander, and external concave reciprocating mirrorset;

FIG. 96 is a variation on the embodiment of FIG. 90 with collimatedlight source, beam expander, and external convex/concave reciprocatingmirror set;

FIG. 97A is a beam-shape transformation element with double Fresnel-typeprismatic beam expansion components,

FIG. 97B shows the detailed angular arrangements of the light rayspassing through FIG. 97A, and

FIG. 97C is a variation on the embodiment of FIG. 90 with a collimatedlight source and prismatic beam expander;

FIG. 98A is a cross-sectional view of a conic refractive beam expanderand

FIG. 98B is cross-sectional view of the collimated reciprocating-mirrorlight source of FIG. 93 with the conic beam-expander of FIG. 98A;

FIG. 99A is a cross-sectional view of an adiabatic beam-shapetransformation and non-imaging collimation system using the converginglight source of FIG. 92 and

FIG. 99B is a perspective view of the light pipe section used in FIG.99A;

FIG. 100 is a collimated unpolarized rectangular light (CURL) sourcevariation based on the reciprocating mirror embodiments of FIG. 93;

FIG. 101 is another CURL source variation based on the embodiment ofFIG. 91;

FIG. 102 is another CURL source variation based on the embodiment ofFIG. 96;

FIG. 103 is another CURL source variation based on the embodiment ofFIG. 96;

FIG. 104 shows a prior art light source polarizer;

FIG. 105 shows a light source polarizer embodiment used with the CURLsources of FIGS. 100-103, a split-image SLM and a projection lens;

FIG. 106 shows a two projection lens variation of the embodiment of FIG.105;

FIG. 107 shows the cross-sectional view of a light source polarizerbased on polarization-converting and selective-reflecting reciprocatingmirrors ;

FIG. 108 shows the cross-sectional view of an embodiment of FIG. 107based on a concave polarization-converting reflector with inlet hole,selective-reflecting plane, composite lens element and collimating lens;

FIG. 109 shows a circular beam-shape variation on the polarizing systemof FIG. 108 based on the converging light source of FIG. 92;

FIG. 110 shows a rectangular beam-shape variation of the polarizingsystem of FIG. 108 based on the collimated light source of FIG. 102 anda condensing lens;

FIG. 111A is a rectangular beam-shape variation on the embodiment ofFIG. 109 using the system of FIG. 96 and

FIG. 111B is a perspective view of the system of FIG. 111A;

FIG. 112 is a rectangular beam-shape variation on the embodiment of FIG.109 using the system of FIG. 98;

FIG. 113A shows a light source polarizer based on a variation of FIG.107 with the polarization-converting reflector hidden in the interior ofa converging unpolarized light beam using a hyperboloidalpolarization-converting reflector and selective-reflecting plane and

FIG. 113B is an alternative embodiment of the quarter wave convertingand reflector elements used in FIG. 113A;

FIG. 114 shows a light source embodiment based on beam expansion and thepolarizing method of FIG. 113 with the beam-expansion method of FIG. 98;also shown is the split polarization beam at the screen;

FIG. 115 is a variation of FIG. 114 with the beam-transformation methodof FIG. 97; also shown is the split polarization beam at the screen;

FIG. 116 illustrates another type of light source system based on thepolarizing method of FIG. 113 with the beam-shape transformation methodof FIG. 98;

FIG. 117 is a variation of FIG. 116 with the beam-transformation methodof FIG. 97;

FIG. 118 shows a collimated light source polarizing variation on FIG.113 and FIG. 32 using reciprocating polarization converting andselective reflecting conicoids;

FIG. 119 is a variation on the embodiment of FIG. 118 for converginglight;

FIG. 120A shows an optimized alignment of a 3M-type selective reflectingfilm sheet when applied to a curved surface and

FIG. 120B shows individual facet portions from an aligned film stock;

FIG. 121A shows a system longitudinal cross-sectional view of apolarized light source variation on the converging light source of FIG.92 with selectively-reflecting conic polarizing element and toricpolarization-converting hyperboloidal converging reflector;

FIG. 121B shows a cross-section along B—B of the output beam of FIG.121A and

FIG. 121C shows a perspective view of the system of FIG. 121A;

FIG. 122 shows a co-axial variation on the embodiment of FIG. 121 forthe collimated light source of FIG. 88;

FIG. 123A shows a light source system using the converging source ofFIG. 92, a negative lens, and the co-axial polarizer of FIG. 122 with avariation on the beam-shape transformation method of FIG. 112,

FIG. 123B shows the transverse beam cross-section taken along B—Bin-between the reciprocating mirrors of FIG. 123A and

FIG. 123C shows the transverse output beam cross-section taken along C—Cof the system of FIG. 123A;

FIG. 124 shows a cross-sectional view of the spatial relationshipbetween the light source reflector of FIG. 92, an SLM and the entrancepupil of the associated projection lens;

FIG. 125 shows a reverse ray-trace method for optimizing the shape of aconicoidal light source reflector of FIG. 124 with ray paths from pupilplane, through an SLM, off a single element reflecting surface and to alight source target zone;

FIG. 126A shows a variation on the method of FIG. 125 for multiple toricreflector segments,

FIG. 126B shows a perspective view of the multiple toric reflectorportion in FIG. 126A and

FIG. 126C shows a Galilean telescope lens system added to the system ofFIG. 126A;

FIG. 127 shows a prior art LCD color-splitting cube used with prior artpolarizing beam-splitter;

FIG. 128 shows the cross-sectional view of a split-image embodiment ofan LCD color-splitting cube with a polarization-selective split-imagecoupler and output beam-splitter for separate projection lenses;

FIG. 129 shows a variation on the embodiment of FIG. 128 for a singleprojection lens;

FIG. 130 shows a variation on the embodiment of FIG. 128 for a singleprojection lens and output polarization;

FIG. 131 shows a variation on the embodiment of FIG. 128 with apost-projection lens beam-splitter;

FIG. 132 shows a variation on the embodiment of FIG. 128 using thealternative polarization-selective split-image coupler;

FIG. 133 is a variation on the embodiment of FIG. 132 using separatepolarization-selective coupling and polarizing methods.

FIG. 134 shows a variation on the embodiment of FIG. 133 using analternative polarizing method and a single projection lens;

FIG. 135 shows a single projection lens variation on the embodiment ofFIG. 128 using two cross-firing LCD color-splitting cubes and integralpolarization-selective and polarizing coupler,

FIG. 136 shows a variation on the embodiment of FIG. 135 forthree-dimensional image projection suitable for use with conventionalfolded-optic rear-projection systems and conventional front projectionsystems;

FIG. 137 shows a variation on the embodiment of FIG. 135 forresolution-doubling split-image projection;

FIG. 138 shows a variation on the embodiment of FIG. 137 for imagecomparison and correlation applications;

FIG. 139 shows a variation on the embodiment of FIG. 137 forthree-dimensional image projection using post-projection lensbeam-splitting and split-image folded-optic projection systems;

FIG. 140 shows a variation on the embodiment of FIG. 139 forresolution-doubling split-image projection using two projection lenses;

FIG. 141 shows a variation on the embodiment of FIG. 140 forthree-dimensional image projection using split-image, two-polarizationfolded-optic projection system and two projection lenses;

FIG. 142 shows a variation on the embodiment of FIG. 128 using two lightsources and a single projection lens;

FIG. 143 shows a variation on the embodiment of FIG. 142 for twoprojection lenses;

FIG. 144 shows a variation on the embodiment of FIG. 142 for singlepolarization split-image folded-optic projection systems;

FIG. 145 shows variation on the embodiment of FIG. 144 for orthogonalpolarization split-image projection systems;

FIG. 146 shows an orthogonal polarization split-image method for thedigital micromirror device (DMD); and

FIG. 147 shows a variation on the split-image projection systemembodiment of FIG. 13 for use with three-dimensional image viewing viathe embodiment of FIG. 141.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An optical system constructed in accordance with one form of theinvention is indicated generally in FIGS. 1A-C and 2 which include aside elevation, front elevation and top elevation. The optical system 10embodies a structure and method which uses various optical elementsdisposed in a compact geometry and manipulates light to generate animage output on an output projection screen 26 The system 10 includes alight source 12 (see FIG. 3) which illuminates a spatial light modulator(“SLM”) 14, such as a conventional liquid crystal display (“LCD”)imaging device or digital mirror modulator (“DMD”). The system can alsobe used with passive image sources such as photographic transparenciesand microfilm. The LCD form of the SLM 14 can be transmissive orreflective. The SLM 14 (LCD or DMD) shown in FIG. 1A, 1C, 2 and 3 isconnected to an appropriate SLM driving circuit 19, consisting ofcontrol electronics 21 and image processing electronics 11, buffered byan associated format memory 9 needed to produce a high qualityblack-and-white or color image (data stream), as shown schematically inFIG. 1D. Electronic video image signals 17 can include, for example,signals from laser disk players, conventional analog television, DSSsatellite television, digital video disk players, video cassetterecorders, personal computers and photo-cd players. The signals 17 areapplied to the electronic pixel addressing structure of the SLM 14 bymeans of an electronic interface 15 that connects to the imageprocessing electronics 11 and the image format memory 9 as shown. Forexample, when one SLM 14 is used for each color component (red, green,blue) as in FIG. 1A and FIG. 3, and/or in situations when multipleimages are applied, the image processing electronics 11 sorts anddirects image information to the correct circuit memory 9 for each SLM14. The SLM 14 can include corrective refractive lens elements, such asconvex refractive lens 16 or concave refractive lens 18 as each memberof a lens pair bracketing the input and output sides of the SLM 14 oreach member located in between the light source 12 and the SLM 14,forming an approximate telescopic unit. A pair of lens locations isshown as dotted lines in FIG. 2. Another pair of lens locations is shownas dotted lines in FIG. 3. These lenses 16 and 18 can improve efficiencyand image contrast under selected optical conditions. In order to secureoptimum performance of a total projection system, as will be developedlater, the light source 12 can involve converging or diverging rays,rather than the nearly collimated rays preferred by the SLM 14. For thisreason, a first lens element 18 can be added to the light source at thefirst location before the SLM 14 where more nearly collimated light isdesired. Then the second lens element 16 can be added at that pointafter the SLM 14 where collimated light is no longer preferred. In caseswhere it is acceptable to use a “telecentric” form of a projection lens20, the use of the second lens 16 is not required. In cases where theuse of a “telecentric” form of a lens is not acceptable, and aconventional form of the projection lens 20 is preferable, the secondlens 16 provides the proper optical power to locate the conventionalprojection lens' entrance pupil. In the case of the conventionalprojection lens rays joining a point on the SLM 14 to the center of thelens pupil make a non-zero angle with the lens axis and are typicallyconverging towards the lens pupil. A conventional “telecentric” lens isone in which these rays can all be parallel to the lens axis. In caseswhere the properties of the light source 12 have been modified, as bythe use of aspheric contour terms that will be introduced hereinafter,either or both of these bracketing lens elements can also be renderedwith aspherizing contours to correctly direct the associated raysthrough the optical system 10. One lens pair that is particularly usefulis the position and negative lens combination that forms an approximatetelescopic unit and placed between the light source 12 and the SLM 14,as will be developed hereinafter, so the angular ray distribution aboutthe principal rays can be more tightly controlled.

As shown in FIGS. 2 and 3, the optical system 10 includes the projectionlens 20 and a beam splitter 22 which routes upper rays 24 having passedthrough one portion of the image of the SLM 14 to an upper image portion86 of a projection screen 26, and lower rays 28 having passed throughthe corresponding region of the image of the SLM 14 to a lower imageportion 88 of the projection screen 26. This arrangement results in theoriginal and complete image being reconstructed in perfect organizationand focus over the projection screen 26. The optical system 10 includesa split-image beam forming system 80 (hereinafter “split image system”)shown in FIG. 2. The split image system 80 includes, for example, atransmissive form of the SLM 14, with an upper image region 82 and lowerimage region 84. Polarized upper rays 24 and orthogonally polarizedlower rays 28 are input to entrance pupil 90 and exit pupil 92 of theprojection lens 20. The beam splitter 22 outputs orthogonally-polarizedupper and lower beams 94 and 96 to the upper and lower image portions 86and 88 of the optical system 10. The split image system 80 is shown ingreater detail in FIG. 3. In this case, the light source 12 is attachedto a polarization selective light source coupler 98 containing an upperand lower diagonal region which allows the light source 12 to be mountedorthogonally to optic axis 100 (side-mounted). The light source 12 isarranged to provide the appropriately polarized upper rays 24 and lowerrays 28 for the upper and lower image regions 82 and 84 of the SLM 14.The resulting upper and lower output beams 94 and 96 (see FIG. 1A) canbe either linearly polarized TE and TM, right and left hand circularlypolarized (RHCP and LHCP) or other available combination which wouldfunction in the illustrated manner. A three color split-image form ofthe SLM 14 includes a conventional sub-assembly 97, containing onesplit-image form of the SLM 14 for each of the well known colorcomponents, namely, red 82R/84R, green 82G/84G and blue 82G/84G lightimages, and the associated color-splitting means. It is the systematicrelationship, however, between the split-image form of the SLMs 14, thebeam splitter 22 and the wide band polarization-dependent nature of thevarious reflecting elements of the optical system 10 which provideimportant advantages.

As shown in FIGS. 1A and 2, the beam splitter 22 is configured toprocess the selectively polarized upper and lower light rays 24 and 28passing through the image of the SLM 14, such that the upper imageregion 82 and lower image region 84 of the SLM 14 are recognized andsorted by their associated orthogonal polarization states for the inputupper and lower light rays 24 and 28. The upper polarized beam 94 andthe lower polarized beam 96 are output and their respective pathsthrough the illustrated optical system 10 depend on and are controlledby the respective orthogonal polarizations. The polarization state givento each of the upper and lower polarized beams 94 and 96 allows theirrespective transmission through upper and lower polarization selectivereflectors 102 and 104 (see FIG. 1A). The continuations of thesetransmitted forms of the upper and lower polarized beams 94 and 96 arepolarization converted and redirected by upper mirror converter 106 andlower converter mirror 108, and back towards the selective reflectors102 and 104. The beams 94 and 96, which have been orthogonally convertedby the upper and lower mirrors 106 and 108, and returned back to theselective reflectors 102 and 104, are redirected towards a Fresnel lens110 and then output for viewing on the projection screen 26.

In the most preferred embodiment, therefore, the upper and lower halvesof the optical system 10 form two identical and symmetric sections. Inthe example shown in FIG. 1A, the upper and lower converter mirrors 106and 108 in combination with the upper and lower selective reflectors 102and 104 are used in each section to apply the respective image portionsonto the projection screen 26.

In a particular form of the embodiment of FIG. 1A, the polarizationselective reflectors 102 and 104 are each tilted at about 42.5 degreeangles with respect to the optic axis 100, and contact, or nearlycontact, the rear of the Fresnel lens 110. The polarization selectivemirrors 102 and 104 are preferably each composed of a rigid andoptically transparent substrate material 112 and 114, respectively, suchas an acrylic or polycarbonate in a coated (or laminated) form. Apreferentially-oriented layer 116 on the reflector 102 and a layer 118on the reflector 104 can both be a wide band selectively reflectingmaterial, such as a well known Minnesota Mining and ManufacturingCompany “Reflective Polarizer”, or the well known Merck Ltd.'scholesteric liquid crystal reflective polarizer “Transmax”. Such wideband reflective polarizers efficiently transmit (and reflect) orthogonalpolarization states over a wide range of angles and wavelengths. For thelinearly polarized embodiment of FIG. 1A, the oriented layer 118 ispreferably the 3M material pre-aligned with the beam splitter 22 totransmit light of polarization state P1 and to reflect light of theorthogonal polarization state P2; the oriented layer 116 is preferablythe 3M Reflective Polarizer pre-aligned with the beam splitter 22 totransmit light of polarization state P2 and to reflect the light oforthogonal polarization state P1. This 3M material is an organicdielectric multi-layer stack which reflects and transmits with nearlyequal efficiency over a very wide band of incident angles andwavelengths. The subject invention can also be practiced usingorthogonal circular polarization states and using the wide band Merckmaterial described above. The well known, more classical inorganicdielectric multi-layer materials perform functionally the same way, butare tuned to a single wavelength, and operate efficiently only over arelatively narrow range of angles. As such, the use of conventionalmaterials is not generally as preferred in forms of the optical system10 which require the display of white light and the ability to handlewith equal efficiency a diversity of angular directions for light.

In the embodiment of FIG. 1A, the corresponding upper and lowerconverter mirrors 106 and 108, each referenced to the back of theFresnel lens 10, are aligned parallel with the optic axis 100, above andbelow by a distance equal to D/2.78. (Note: D is the diagonal of theprojection screen 26, and D′ is the height of the projection screen 26so that D′=(3/5)D for the standard 4:3 TV aspect ratio.) In theconstruction of these embodiments, there are many combinations of mirrorheights above the optic axis 100 and source locations that provide thecorrect output angles to the Fresnel lens 110. Additional criteria forthe preferred location involve making sure that the optical path lengthof the ray directed to the center of the projection screen 26 divided bythe cosine of the angular range equals (or nearly equals) the opticalpath length of the uppermost ray. Moreover, rays from the top, middle,and bottom of the exit pupil 92 of the projection lens 20, through thebeam-splitter 22, should arrive at the projection screen 26 at the same(or substantially the same) physical point. The embodiment conditionsthat best satisfies these aggregate conditions will be preferred forhighest projected image quality (focus) on the projection screen 26without correction or compensation accessories. Embodiments that failthese conditions by large amounts will result in blur circles on theprojection screen 26 exceeding the resolution as defined by themagnified pixel element size on the screen and will generally beimpractical. The example of FIG. 1A is within the preferred range, butnot necessarily the optimum condition. Other examples, failing thesecriteria can be corrected by the use of additional elements and broughtwithin the range of preference. Each of the upper mirrors 106 and 108preferably also contains two layers, one a wide band mirror layer 120(typically a metal or metal-like film that changes the handedness ofcircularly polarized light, from right hand circular to left handcircular, or vice versa) and another, a wide band polarizationconverting layer 122, preferably a wide band quarter-wave retardationfilm. A preferred wide band polarization converting material is wideband retardation film manufactured by, for example, Nitto DenkoCorporation, Japan, which supplies essentially the same phaseretardations at any wavelength between about 400 nm and 700 nm.Conventional retardation materials designed for a particular centerwavelength exhibit progressively larger retardation errors the furtherthe operating wavelength differs from the center wavelength in eitherdirection.

In the illustrated embodiment of FIG. 1A, each of the upper mirrors 106and 108 preferably also contains two layers, one a wide band mirrorlayer 120 (typically a metal or metal-like film that changes thehandedness of circularly polarized light, from right hand circular toleft hand circular, or vice versa) and another, a wide band polarizationconverting layer 122, preferably a wide band quarter-wave retardationfilm (see FIGS. 4A and 4B). Conventional retardation materials designedfor a particular center wavelength exhibit progressively largerretardation errors the further the operating wavelength differs from thecenter wavelength in either direction.

The beam splitter 22 in FIG. 1A is preferably placed along the opticaxis 100 in the vertex formed by the upper and lower selectivereflectors 102 and 104, nominally a distance D/30 from the back surfaceof the Fresnel lens 110. Again, as described hereinbefore, there aremany combinations of source and mirror location which result indifferent D values. The projection screen 26 and the Fresnel lens 110are positioned in a plane substantially perpendicular to the optic axis100 and are almost in physical contact, contrary to the exaggerated viewshown for clarity in the illustration of FIG. 1A. The projection lens 20is assumed as f/2.5 with a 0.5″ focal length set by the SLM's 14presumed 0.7″ diagonal aperture and the lens' +/−35 degree angularcoverage and an entrance pupil of 5 mm. The corresponding angular extentof the upper and lower beams 94 and 96 is therefore 22.8 degrees in airfor the side-view angle A of FIG. 1A, an angle of 29.2 degrees (notshown) which corresponds to the angular extent in the horizontal planeof FIG. 1B, and a angle of 35 degrees (not shown) in the plane of thediagonal D, as indicated in FIG. 1B.

A conventional prior art system 124 shown in FIGS. 5 and 6 uses a 45degree folded design for a mirror 126 and achieves a depth D/2.23 for a52 degree fill angle projection lens beam, where D is taken as thescreen diagonal. The projected image is true to the original, which isto say there is neither any shape distortion known as “keystoning,” orde-focusing. Keystone distortion occurs when the sides of the image arebent either in towards the center or out from the center, creating ashape reminiscent of an architectural keystone. When the projection lens20 f/# is decreased so as to widen the projection angle to +/−35degrees, cabinet depth, t, is reduced to D/2.4, also without keystoning.Steepening the folding mirror angle from 45 degrees to 60 degrees andkeeping the 70 degree lens reduces cabinet depth, t, still further toD/3.3, but introduces a significant degree of keystone distortion. Todate, the best commercially available rear projection cabinet depth, t,is about D/2.5 and requires space to store the illumination and basicimage-forming components (the light source 12, the SLM 14, theprojection lens 20 and the beam splitter 22) in a sub-cabinet 15 belowthe projection screen 26, as shown in FIG. 6. The minimum cabinetdepths, t, for state-of-the-art, commercially available 50″ diagonalrear-projection television systems are about 20″, with sub-cabinetheights of about 12″-24″.

The invention of FIG. 1A and its associated variations, on the otherhand, achieves a depth, t, that for preferable arrangements andembodiments is between D/4.4 and D/4.8 (as in FIG. 1A, with noassociated keystone distortion). The design as shown in FIG. 1A fitswithin D/4.6 using a tilt angle of 43 degrees to the optic axis 100.Other variations allowing a correctable amount of keystone and otherdistortions can be made to fit within a depth of D/4.8 or better. Suchresults can be obtained with only a partial folding-mirror cabinetextension, e, needed above and below (or equivalently to the left andright) of the projection screen 26. The image is projected flush to eachof two opposing viewing edges 130 and 132, in FIG. 1A. Other variationson FIG. 1A, to be described hereinafter, require no cabinet extensionswhatsoever and exhibit substantially borderless viewing on all fourviewing screen sides, enabling their use in arrays.

A computer program (microfiche Appendix 1: FOLD2) can be used to analyzeall possible arrangements of reflecting elements for the embodiment ofFIG. 1A, in terms of differences in optical path length, degree ofkeystone distortion and practicallity of projection lens andbeam-splitter locations. The results of this program were then used todetermine the minimum value of cabinet depth, t, for a practical design.While the use of this program can be helpful, proper variations on FIG.1A can be readily designed manually using the principles describedherein.

To further illustrate operation of the preferred embodiment of FIG. 1A,consider upper ray 134 from the upper beam 94 exiting the upper portionof the beam splitter 22 placed on the optic axis 100 just inside theapex formed by the reflectors 102 and 104. The polarization state, P1,of the upper ray 134 is established by the beam splitter 22. The upperray 134 proceeds upwards at an inclination angle to the vertical that isapproximately 30 degrees and passes through the polarization selectivereflector 102, which is essentially transparent to light in thepolarization state P1. As shown in detail in FIGS. 4A and 4B when theupper ray 134 reaches the upper converter mirror 106, it first passesthrough the transmissive converting layer 122 and is converted to righthand circular polarization (RHCP). The upper ray 134 then is reflectedat the surface of reflective converter mirror layer 120, a process thatchanges the ray's direction and converts its state of polarization fromRHCP to LHCP. The reflected upper ray 134 passes back through thetransmissive converting layer 122, which converts its state ofpolarization to P2 as output upper ray 140, heading back towardspolarization selective reflector 102, but displaced significantly to theright from its first point of entry. As shown in FIG. 1A on striking toplayer 116 of the polarization selective reflector 102, the upper ray140, now polarized as P2, is reflected as processed ray 144 heading leftto right towards the top of the Fresnel lens 110 at approximately a 23degree angle with the optic axis 100. When this ray 144 actually reachesthe Fresnel lens 110, it is redirected along the optic axis 100 byFresnel facets, so that the ray 144 reaches the projection screen 26 insharp focus and is made parallel to the optic axis 100 and directed tothe viewer.

In this manner, one half of the image is presented on the upper imageportion 86 of the projection screen 26, and the other half of the imageis presented on the lower image portion 88 of projection screen 26. Theimage portions 86 and 88 mesh together precisely on the projectionscreen 26 by virtue of a sharp vertex formed by the top surface layer116 and the bottom surface layer 118 of the polarization selectivereflectors 102 and 104 combined with the micro-alignment of the beamsplitter 22 along the optic axis 100. Optionally, this can beaccomplished by the micro-tilt of any one of the four major foldingmirrors 102, 104, 106 and 108, so there is no visible separation line atthe boundary between the upper and lower image portions . Thisadjustment becomes especially important if there is any deliberatelyformed gap or buffer zone 148 between the SLM image portions, as shownin FIG. 2. The primary methods for making the needed adjustment involvesphysically shifting the beam splitter 22 laterally along the optic axis100, or by adding a slight tilt to the upper and/or lower foldingelements (the upper and lower converting mirrors 106 and 108). Sincethese converter mirrors 106 and 108 are preferably horizontally alignedand mounted to the top (and bottom) of the cabinet, the use of setscrews is particularly easy.

Should the embodiment of FIG. 1A result in inversion of the orientationof each image half of the SLM 14, so that they are applied to theirrespective halves of the projection screen 26, upside down, electroniccorrection means can be made in which the LCD or DMD form of the SLM 14organizes the image pixels in a proper manner.

In FIGS. 7 and 8 are illustrated another variation of the invention ofFIG. 1A. In this embodiment the selective reflectors 102′ and 104′ arecurved, rather than planar. The principal of this embodiment isillustrated in FIG. 8 by superimposing the rays and elements of the twoapproaches. As in FIG. 1A the upper polarized beam 94 is shown asemanating from point 150 on the optic axis 100 and reflecting back fromthe upper converter mirror 106 as if the light were actually emanatingfrom virtual point 152 (see FIG. 8). Output ray 154 makes a 23 degreeangle with the optic axis 100 after re-direction by the mirror 102 as ifit had emanated from point 156. Had this ray 154 appeared to emanatefrom point 158, it would be ray 154′ and its output angle would be 35degrees; and the space between the top of the projector screen 26 andthe mirror 106 would be illuminated fully. Achieving this change inbehavior for the ray 154′ is possible by giving the mirror 102′ ahyperboloidal curvature as for the mirror 102 with one focus at thepoint 152 and the other at the point 158, rather than 156. The benefitof this variation is that it allows a more compact arrangement of theelements, fitting within a cabinet depth of D/5.4, rather than D/4.6.While no keystone distortion is involved in the altered design, theprojection lens 20 is modified to operate under these conditions wherethere is a small difference in optical path length from the center ofthe projection screen 26 to the edge. Alternatively, aspherizing termscan be added to the hyperboloid surface function to compensate for thepath length differences. Other related variations include the caseswhere the converter mirrors 106 and 108 can also be curved rather thanplanar, and where all the mirrors 102, 104, 106 and 108 are curvedrather than planar. In these cases, mirror 106′ shown in phantom (andits companion 108′; not shown) in FIG. 8 sloping upwards, and themirrors 102′ and 104′ are sloping downwards from the planar mirrorembodiment of FIG. 1A.

In a variation shown in FIG. 9, the upper converter mirror 106 and thelower converter mirror 108 are tilted and also the input beam locationsare moved progressively back to the rear of the cabinet. This embodimentachieves a depth of D/4.9. The angle made by each of the selectivereflector mirrors 102 and 104 with respect to the optic axis 100 isfurther increased from 42.5 degrees in the embodiment of FIG. 1A to 45degrees in FIG. 9. In addition the tilt angle with respect to thehorizontal of the converter mirrors 106 and 108 is 15 degrees.

Using the upper polarized beam 94 of polarization P1 as an example,consider in FIG. 9 the paths of illustrative ray 206 and the upper ray134 in the upper beam 94. Each of these rays travel upward and passesthrough a transparent substrate 186 of the mirror 102 and its reflectivetop surface layer 116 (see magnified detail of FIG. 9) in sequenceheading towards an upper converter mirror 106. On reaching the upperconverter mirror 194, each of these rays 134 and 206 experiencepolarization conversion and redirection in the manner shown in FIG. 4B.Each of the rays 134 and 206 passes first through the quarter-wavetransmission converting layer 122, preferably a wideband quarter-waveretardation film, and is efficiently converted to right hand circularpolarization. Each of the rays 134 and 206 then strikes the surface oflayer 120, whereupon they are converted to their orthogonal state ofcircular polarization, in this case left hand circular polarization, andis redirected downwards and back towards the upper selective reflector102. So directed, each of the rays 134 and 206 then passes back throughthe transmission converter layer 122, and becomes polarized to P2, whichis of orthogonal linear polarization to P1. These rays 134 and 206 nowreflect from the selective reflecting layer 116 on thetransmissive/reflective substrate 186, and are redirected to the leftand towards the Fresnel lens 110 and the upper half of the projectionscreen 26. Therefore in more detail, the extreme upper ray 134 firstpasses through the upper selective reflector 116 as ray 208, re-strikesthe reflector 116 as the orthogonally polarized ray 210, and isredirected as output ray 212 at an oblique angle to the optic axis 100at the uppermost output point in the optical system 10. The Fresnel lens110 in this region is designed to redirect the output ray 212 so itreaches the top of the projection screen 26, nominally parallel to theoptic axis 100. A central ray 214 travels in a direction perpendicularto the plane of the upper converter mirror 106. As such, it is convertedto polarization P2 as before, but reflected back on itself as ray 216returning towards the layer 116 of the upper selective reflector 102. Asin the previous manner, the ray 218 is selectively reflected at thelayer 116, and redirected towards the central portion of the Fresnellens 110, where its ray direction is made normal to the central portionof the projection screen 26. A second extreme ray 206 passes through thetop surface layer 116 and its transmissive substrate 186 as ray 222,reaching the left-most edge of the upper converter mirror 106, whereuponit is converted and redirected, as above, as downward extreme ray 224.This downward extreme ray 224 strikes the left-most edge of thereflective layer 116, and is redirected perpendicularly to the Fresnellens 110 as ray 226. This ray 226 represents the lowest pixel row in theupper image region 82, and is applied to the center of the projectionscreen 26.

Another related embodiment is illustrated in FIG. 10 for the case wherethe input beams are moved closer to the rear surface of their respectiveupper and lower selective reflectors 102 and 104, and the use of twoseparate projection lenses 20, one at an upper point 228 and another ata lower point 230. This embodiment includes locating the beam splitter22 at the output side of the SLM 14 rather than at the output side ofthe projection lens 20 as was the case above. The advantage of thisapproach is an additional reduction in cabinet depth, t, to D/5.0.

The reductions in cabinet depth shown in FIGS. 7-10 are a directconsequence of the hyperbolically-curved reflecting elements.Industry-standard raytrace software program, ASAP, as supplied byBreault Research Organization, was used to develop scale-models forvarious designs. Hyperbolic curvatures were selected that the achievedthe same proper output ray angles at the projection system 10 in FIG.1A. For example, consider the case of the hyperboloidal selectivereflector 102 in FIG. 7. One focus, F_(b), was set back on the system'soptic axis 100 a distance sufficient to create the maximum desiredoutput angle for the rays at the top (and bottom) of the Fresnel lens110, in this case 35 degrees. The other focus, F_(f) was iterativelyplaced along a vertical line extending directly above the source point.The line connecting the two foci defines the axis of the hyperboloid.The actual height of focus F_(f) was adjusted so that the output rays atthe center of the Fresnel lens 110 arrived at normal (or near normal)incidence. For the example of FIG. 7, this hyperboloid has focireferenced to the system origin (at the vertex point of the two tiltedselective reflectors 102 and 104) of (−D/2.6, 0) and (−D/42, D/1.67)._Any equivalent commercial raytracing program, including Code VA andSuper Oslo, can be used for the same purpose.

Another form of the invention is shown in FIG. 11, and this embodimenteliminates the need for the protruding extension zones, e, shown in FIG.1B. In this embodiment, the symmetrically arranged upper and lowerselective reflectors 232 and 234 are now tilted away from, rather thantowards, the Fresnel lens 110, and upper converter mirror 236 and lowerconverter mirror 238 lie in the upper and lower horizontal planes as inFIG. 1A, as opposed to being tilted away from this plane, as in FIG. 9and FIG. 10. The mirror 236 (and, by analogy, the mirror 238) serves asa mirror plane for a light source (not shown) on the optic axis 100disposed at point 240 but located at virtual point 242. Instead of firstpassing through the polarization selective reflectors 102 and 104, thisembodiment starts with the orthogonally polarized upper and lower beams94 and 96 from the beam splitter 22 and first striking the upper andlower reflectors 236 and 238. These reflectors 236 and 238 redirect thebeams 94 and 96 from their starting point on or near the optic axis 100.Once redirected, the beams 94 and 96 pass through the first selectivereflector 102 or 104 encountered, and then are redirected towards theprojection screen 26 by the appropriate selective reflector 232 or 234encountered.

Consider the illustrative path of central ray 244 through the foldedoptical system 10 of FIG. 11. This ray 244 of polarization state P1leaves the upper output face of the beam splitter 22 and is so directedtowards the upper converter mirror 236 shown in FIG. 11. The ray 244 isthen redirected by the mirror 236 and through the selective reflector232 as ray 248 and is then reflected as ray 257 by theorthogonally-aligned reflector 234 towards the Fresnel lens 110. TheFresnel lens 110 acts upon all incident rays so they are parallel, ornearly parallel, to the optic axis 100. This process occurssymmetrically in reverse for lower ray 252 to output a ray 255. Thisarrangement applies the upper image to the lower portion of theprojection screen 26 and the lower image to the upper portion of theprojection screen 26. An image orientation correction can be madeelectronically within the SLM 14, as previously mentioned, so that thistransform reconstructs a perfectly organized image. Clean-up filterdevices, to be described hereinafter, can also be applied, for example,on the output faces of the beam splitter 22 of FIG. 11, or can belaminated to the upper and lower converter mirrors 236 and 238, or canbe laminated to the upper and lower portions of either the projectionscreen 26 or the Fresnel lens 100.

Another embodiment of the invention is shown in FIG. 12 that preservesthe image orientation. In this case a thin two-sided,polarization-converting mirror plane is inserted on the optic axis 100,symmetrically in between the upper and lower portions of the opticalsystem 10 of FIG. 11. Upper image rays 254 of polarization P1 outputfrom the beam splitter 22 remain in the upper image region 86 of theoptical system 10 and are applied to the upper portion of the projectionscreen 26. In one embodiment, a plane mirror 256 contains, on each topand bottom side, an outer layer of wide band polarization convertingmeans, preferably a quarter-wave retardation film 122, like the wideband converter layer of FIGS. 4A and 4B. The upper image ray 254 leavesthe upper portion of the beam splitter 22 in polarization state P1, isredirected downwards by the upper mirror 236 as ray 258, also inpolarization state P1. This ray 258 is able to pass through the upperselective reflector 232 which passes P1 and reflects P2. When the ray258 reaches the vicinity of the plane mirror 256, it first passesthrough the converter layer 122, whereupon it is converted to RHCP,reflected from the plane mirror 256 as LHCP, and output as ray 260 inpolarization state P2 as before heading back towards the upper selectivereflector 232. On reaching the reflector 232, the ray 260 now orthogonalin polarization to the previously transmitted ray 258, is redirectedtowards the Fresnel lens 110 and then the projection screen 26 asbefore. Alternatively, and with substantially the same effect, theretardation film 122 on the reflecting plane mirror 256 can be relocatedon the bottom and top side, respectively, of the upper mirror 236 andlower mirror 238, respectively. In either case, light rays that havepassed through the upper image region 82 of the SLM 14 are applied tothe upper portion 86 of the projection screen 26, and light rays thathave passed through the lower image region 84 of the SLM 14 are appliedto the lower portion 88 of the projection screen 26.

In the embodiments of FIG. 11 and FIG. 12, as drawn, a cabinetthickness, t, is D/3.2, and neither requires keystone correction.Improved compactness can further be achieved by at least one (1)steepening the tilt angles of the upper and lower selective reflectors232 and 234, and (2) shaping one or both of their reflecting surfaces ofthe reflectors 232 and 234, or (3) by shaping the upper and lowerconverter mirrors, 236 and 238.

One such variation on the embodiment and method of FIG. 11 and FIG. 12,using curved rather than plane redirecting mirrors, is shown in FIG. 13.In this embodiment symmetrically disposed, selective reflector elements262 and 264 are tilted more steeply (35 degrees from the vertical) thanin either FIG. 11 or FIG. 12 (47 degrees from the vertical), making fora correspondingly more compact arrangement. The horizontal, upper andlower mirrors 236 and 238 of the previous embodiments are thus replacedby curved reflectors 266 and 268. These reflectors 266 and 268 arepreferably hyperboloidally shaped, with foci for both of the upper andlower curved reflectors 266 and 268 located at virtual source points 270and 272, and points 274 and 276, respectively. The curved reflectors 266and 268 are shaped to redirect all rays from source apertures whosecenters are located at the points 272 and 276, as if the source aperturewere really centered at the points 270 and 274, respectively. Thefurther the virtual source points 270 and 274, are displaced from theoptic axis 100, the steeper can be the tilt angle of the selectivereflector elements 262 and 264. The cabinet depth, t, for the particulararrangement drawn is improved to D/4, and uses the less demanding 52degree projection lens 20.

Yet another preferred embodiment of the above methods in FIG. 14Ainvolves steepening the tilt angles of the polarization selectivereflector 102 in FIG. 1A to 90 degrees, so as to form, instead, avertical selective reflector 277 and then simultaneously re-positioningthe corresponding polarization-converting folding mirror 282 so as to betilted to the vertical back cabinet wall at an angle, ψ, so that the topedge of the mirror 282 moves closer to the projection screen 26. Theseelements can be arranged to fit within a cabinet depth, t, of D/n, wheren is between 4.5 and 5.5. This embodiment achieves important advantagesover conventional tilted-mirror folded-optic systems that have dealtwith polarized light. The present embodiment, as in FIG. 1A, uses a moreefficient polarizing beam splitter material, not in its conventionalbeam-splitting manner, but rather more efficiently as a selectivetransmitter (or reflector) arranged to transmit or reflect incidentlight depending on the linear or circular polarization state applied.Improved efficiency derives from this mode of operation and the factthat the transmissivity or reflectivity is constant (or nearly constant)over a wide range of angles and wavelengths by virtue of using the 3Mand/or Merck materials described hereinbefore. The present embodimentalso uses a two layer structure for the folding mirror 282 (the mirrorlayer 120 and the converting layer 122) to simultaneously convertpolarization from one linear or circular polarization state to theorthogonal state, over a wide range of angles and wavelengths. In FIG.14B is also shown another variation on the embodiment of FIG. 14A wherecentral ray 201′ first strikes folding mirror 282′ rather than selectivereflector 277, a two layer structure is used for the selective reflector277′ (the selective reflector 277 and the converting layer 122) and asingle layer structure is used for the folding mirror 282′ (polarizationconverting metal or metal-like mirror layer 120). Moreover, in thisarrangement, the central input ray 201′ is pre-converted as right-handcircular polarization. As such, in the embodiment of FIGS. 14A and 14B,substantially all light is either reflected or transmitted, and noadditional mechanical devices are needed to deflect any appreciableportion of this light from passing through to the projection screen 26.In addition, principal ray 201 (201′ in FIG. 14B) from the center of theimage to be projected is arranged specifically by the relative anglesbetween the reflector 277 (277′ in FIG. 14B) and the folding mirror 282(282′ in FIG. 14B) and their corresponding slopes causing reflection, sothat its folded path causes arrival of the principal ray 201 (201′ inFIG. 14B) at normal (or nearly normal) incidence to the Fresnel lens 110and the plane of the projection screen 26. Angular deviations of thisray 201 (201′ in FIG. 14B) from normal cause, as previously discussed, aform of image distortion known as keystone distortion to be consideredin more detail later. Moreover, the optical path lengths of extreme rays293 and 295 in FIG. 14A (or 293′ and 295′ in FIG. 14B) are balanced withthat of the central principal ray 201 (or 201 in FIG. 14B) according tothe following equations:

for the upper portion 86 of projection screen 26,$\frac{\left( {{ab} + {cb} + {c\quad d}} \right)}{\cos \quad \theta_{2}} = \left( {{ae} + {ef} + {fg}} \right)$

for the lower portion 88 of projection screen 26,$\frac{\left( {{ab} + {cb} + {c\quad d}} \right)}{\cos \quad \theta_{3}} = \left( {{ah} + {hi} + {ij}} \right)$

Small differences between the left hand and right hand sides of theseequalities are allowed provided they are properly compensated withappropriately disposed refractive elements. For example, see FIG. 73,and further details will be provided hereinafter. The same analysis isapplicable to the alternative arrangement of FIG. 14B.

In the method of FIG. 14A (and 14B), the image source is virtuallylocated at point 288, and sequentially folded first to virtual point 290by the tilted folding mirror 282, then to virtual point 288 (also markedas a) by the vertical polarization selective reflecting plane 277, andthen to real point 286 (also marked as a′) by vertical folding mirror283.

Another embodiment includes that of FIG. 15 which fits within a cabinetdepth, t, of D/4.9. The plane folding mirror 282 used is tilted to thevertical by about 19 degrees. The same projection conditions are appliedas in the embodiments above. In this case, the minimum possibleunder-cabinet depth, t, is about D/11. As before, the folding mirror 283can be applied to reduce cabinet depth, t, by moving the source pointfrom 286 to 288.

As shown in FIG. 16, an additional form of the system 10 in FIGS. 14 and15 can use a slightly curved, rather than planar,polarization-converting reflector 290 with the slight curvatureincreasing compactness still further. The physical curvature is soslight that its presence is shown by comparison with line 292 drawnthrough the source point 286. The nature of the curvature is magnifiedand exaggerated in the detail of the reflector 290 shown to the right.In this example, a hyperboloidal function is used with its two foci (notshown) lying at points in front of (to the left of) and behind (to theright of) the curved reflector 290. This variation is analogous to thatin FIG. 7 above. The particular arrangement of FIG. 16 also uses thefolding mirror 283 to fit within a cabinet depth, t, of D/5.3.

The various embodiments of FIG. 15 are distinguished from the precedingforms in that the upper and lower input beams, such as 94 and 96, in thepreceding figures are now combined into a single beam 97 and processedon their first encounter with the vertical selective reflector 277, bythe action of reflection rather than by selective transmission. Thecentral principal ray 201 in FIG. 14A represents the center of the imageand is folded to the center of the projection screen 26. The lowerextreme ray 295 of FIG. 14A corresponds to the bottom of the lower imageportion 88. Together the bundle of angles between the principal ray 201and the extreme ray 295 are equivalent to the lower beam 96 in FIG. 1A.Therefore, upper extreme ray 293 represents the top of the upper imageportion 86. The bundle of angles between the upper extreme ray 293 andthe central principal ray 201 is equivalent to the upper beam 94 in FIG.1A. By so combining the upper and lower image beams 94 and 96 into beingadjacent, nearly equivalent compactness can be achieved with theasymmetric form of the system 10 of FIG. 14. It is a consequence of thiscondensed condition, however, that the source aperture is locatedbeneath, rather than behind, the final redirecting element. Preciseimaging practice requires that output rays from the center of the imagefield must be made parallel to the optic axis 100, a condition that wassatisfied in previous examples by effectively positioning the sourcepoint (e.g., the point 272 in FIG. 13) behind the operative final outputreflector (e.g., the selective reflector element 264 in FIG. 13). In theadjacent beam embodiments of FIG. 14, a steeper and more compact foldingmirror arrangement, the further below the optic axis 100 the source 12should be offset (e.g., the source point 286 in FIG. 14).

In FIG. 14A, the principal ray 201 is arranged to strike the verticalselective reflector element 277 prior to striking the plane foldingmirror 282. The reverse condition, in FIG. 14B, where the ray 201 isdirected to strike these elements in reverse order, is also possible.Moreover, a preliminary folding mirror 283 can be added to the cabinet'sback-plane, as previously indicated, to relocate the source point morecompactly from a to a′ or the point 288 to the point 286 in FIG. 15. Theillustrative principal ray 201 in FIG. 14 is now redirected by theselective reflecting element 277 towards the folding mirror 282 as ray296, and then converted and redirected by the action of the foldingmirror 282 as output ray 298. For example, the right-hand polarized ray201′ in FIG. 14B can also be directed at first towards the tiltedpolarization (handedness) converting folding mirror 282′ and redirectedas a left-hand circularly polarized ray segment towards the selectivereflector 277′ (now comprising preferably the quarter-wave convertinglayer 122 and the polarization-selective reflector 277). The ray isreflected by the reflector 277′ back towards the tilted mirror 282′ in astate of left-hand circular polarization, which subsequently converts toright-hand circular polarization on re-direction at the mirror 282′, andthen is able to pass through the reflector 277′ on its return. Thisreverse approach of FIG. 14B does not decrease cabinet depth more thanFIG. 14A and requires somewhat more under-cabinet space than thearrangement of FIG. 14A.

The embodiments of FIGS. 14-16 assume a linearly polarized form of theprincipal ray 201. The same results are obtained, however, if the ray201 is circularly polarized (i.e., LHCP source beam 300 as in FIG. 17using the previously described 3M-type material as the selectivereflector 277 and RHCP source beam 302 as in FIG. 18 using thepreviously described Merck-type material as the selective reflector277). In FIG. 17, the converting layer 122 is moved from the left sidesurface of the folding mirror 282 to the right side surface of thevertical selective reflector 277. By this modification, the LHCP inputray 300 is converted to P2 by its passage through the quarter-waveconverting layer 122 and thereby reflected by its initial contact withthe 3M-type linear polarization selective reflector 277. Then, reflectedray 304 is redirected back through the converting layer 122 towardsmetal reflector 306 (such as a metal reflector like the layer 120described hereinbefore) and returned to RHCP. After reflection at themetal reflector 306, return ray 308 is RHCP and becomes P1 on passagethrough the layer 122 at the selective reflector 277 and is thentransmitted efficiently as ray 310.

Another embodiment is described in FIG. 18 using the Merck-type materialas the selective reflector 277. In this case, no polarization convertingmeans other than the tilted metal reflector 306 is utilized. The sourceRHCP ray 302 is reflected by the cholesteric (Merck-type) selectivelyreflector 277 and redirected as RHCP ray 312 to the metal reflector 306,whereupon it is converted to LHCP and redirected back towards selectivereflector 277 as redirected LHCP ray 314. On reaching the reflector 277the LHCP ray 314 is efficiently transmitted and output as ray 316.

Another computer program (microfiche Appendix 2: FOLD) was developed toanalyze in the same way as with FIG. 1A, the example of FIG. 14A (or B)to determine the optimum conditions for tilt angle, angular extent andpractical position for the light source 12. The limiting depth, t, forthis particular case is found to be D/5.19 for a tilt angle of 22degrees and a 60 degree source; D/4.74 for a tilt angle of 18 degreesand a 52 degree source. The result illustrated in FIG. 15 allows a smallamount of correctable keystone distortion.

The embodiments of FIGS. 17 and 18 can be extended in FIG. 19 to thecase where polarization selective reflector 102 and a non-selectivereflecting mirror 318 are arranged in parallel with each other, andwhere source rays 320 enter the optical system 10 through a smallphysical hole 322 the size of the projection lens' exit pupil 324 (0.2″in the above examples) cut in the double-layer 318 including the mirrorlayer 120 and the wide band polarization converting layer 122. In thiscase, the cabinet depth, t, is D/4.8 for the +/−35 degree projectionlens 20 considered above. There are two performance issues associatedwith preferred embodiments based on this approach wherein (1) thelow-angle image source rays 320 are prevented from escaping back outthrough the physical hole 322 upon retro-reflection from the selectivereflector 102, and (2) the absence of image information within a holeprojection region 326 on the projection screen 26 preferably iscorrected.

Still further improvement is possible by adding optical power to theback-reflecting plane preferably in the form of a convex hyperboloidalcurvature reflector 328 (composed of the mirror and converting layers120 and 122), as shown by way of cross-section in FIG. 20. The maximumpractical compactness in this case is a cabinet thickness, t, of D/5.8,when the edge or extreme rays are 37 degrees from horizontal at the rearof the Fresnel lens 110. Somewhat less improvement is possible whenusing a hyperbolic cylindrical curvature rather than the rotationallysymmetric system. In either case, the cabinet-depth is determined by ascale-model made using a commercial raytrace program such as mentionedhereinbefore. The set of hyperboloidal foci in the example of FIG. 20are F1 at −D/4.86 and F2 at D/2.81. The embodiments based on FIG. 19 aredistinguished by the fact that input source rays, such as those bound bythe ray 320, enter the optical system 10 through the physical hole 322formed in the otherwise opaque reflector 328. The embodiments of FIG. 1Aand FIG. 14 each allowed input light to be transmitted through theselective mirror layer (102 in FIG. 1A and reflector 277 in FIG. 14)only after an initial blockage by that selective mirror layer due to thelight being in a reflecting rather than transmitting polarization state.The embodiments of FIG. 14 allowed input source rays (i.e., the ray 293in FIG. 14) to enter the optical system 10 from beneath the variousreflector (the reflector 277 and the folding mirror 282).

One consequence of inputting the bundle of source rays bounded by theedge rays 320 through the physical hole 322 is that some imageinformation can be lost by inadvertent low-angle return-reflections thatpass back through the physical hole 322. Minimizing such losses impliesmaking the hole 322 as small as possible, and/or developing other meansof assuring that no important image information can be sacrificed inthis way. The minimum hole diameter corresponds to that of the exitaperture of the projection lens 20 which in the previous examples hasbeen 0.2″. One other consequence of passing the image source rays bundlethrough the hole 322 is that without some means of compensation orcorrection, the hole 322 is likely to appear on the projection screen 26as an absence of image information.

One method and system for preventing loss of the low angle image sourcerays back through the rectangular physical hole 322 is by prearrangingthat no image information is contained within ray angles small enough toescape, or that only “black rays” (no rays with any image information)are contained in such escape angles. The limiting ray angles for thismethod are shown in FIG. 21 for the illustrative case when D is 20″. Thehalf-angle, A, within which there must be no image information, or onlyso-called “black rays”, is 0.57 degrees, or [ARCTAN a/D] where theparameter “a” is the diameter of the projection lens exit pupil.Accordingly, one can construct the central SLM 14, buffer zone 148analogous to that arranged in FIG. 2. This buffer zone 148 assures thatany low-angle rays that do escape back through the physical hole 322 doso without sacrificing any valuable image content. This buffer zone 148can either be formed as a circular (or rectangular) central region or itcan be arranged as a stripe separating the upper and lower imageportions 86 and 88. The reason for the original buffer zone 148 of FIG.2 was to avoid cross-contamination of rays from the upper and lowerimage portions 86 and 88 of the SLM 14 being misplaced on the projectionscreen 26. The same approach is extendible to the embodiment of FIG. 20,by programming those specifically illuminated pixels within this range,for example, the central +/−0.57 degrees of light from the light source12, to contain no image information other than blocking the transmissionof light, and then to transform the location of image pixels so thatwhen an optical means is subsequently applied to collapse the holeprojection region 326 (see FIG. 20) at the projection screen 26, aperfectly arranged and uniform rectangular image results.

In more general terms, the embodiment of FIG. 21 can be describedanalytically in terms of the pupil diameter, the screen diagonal, D, aprojection lens half angle (as in the above examples) of 35 degrees, ahalf angle of A for the first image-light-containing principal ray 330closest to the optic axis 100, and a separation distance, d, from thehole 322 to output plane. On its first encounter with the projectionscreen 26 hence point 332, the diameter of the ray bundle is 2a/3. Ifthe black area on the projection screen 26 is chosen with diameter A,then: ${\tan \quad A} = \frac{5a}{6d}$

the principal ray 330 directed along angle A meets reflecting surface334 at a height 2(d) tanA or 5a/3 with an upward slope of A. If theprincipal ray 330 is to become the ray arriving at the center of theprojection screen 26, its upward angle A is to be converted to adownward angle 5a/3d. This condition can be achieved by tilting themirror 318 along line 336 which is inclined to the vertical by0.5(A+5a/3d). For small angles, tan A=A (in radians) therefore the tiltangle is 1.25(a/d). For the case of a 50″ screen diagonal and a 0.2″pupil diameter and a projection half angle of 35 degrees, d is 11.9″ andthe tilt angle is 1.2 degrees.

If the opaque area on the projection screen 26 is a circular disk, thenthe tilt angle given corresponds to reflecting mirror 318 being formedas a very shallow cone, rather than the flat plane of FIGS. 20 and 21.If the opaque area is a narrow strip, then the tilt angle A, given aboveis applied to the upper half 318A and lower half 318B of the mirror 318,as in FIGS. 22 and 23, respectively.

One approach for collapsing this dark projection region 326 created onthe projection screen 26 is by means of a beam displacement method. Onemeans of beam displacement is to tilt or otherwise shape (as above) thenon-selective reflecting mirror 318 in FIG. 21 so that, for example, itsnew reflecting surface 318′ deflects the ideal principal ray 330 fromthe normal target point 332 to deflected target point 338 on the opticaxis 100.

In another form of the invention one can collapse the dark projectionregion 326 (see FIG. 19) by covering the physical hole 322 with apolarization selective reflecting material, and arrange elements so thatany returning rays will fail the protective material's condition forescape via transmission back through the hole 322. Doing so, however,requires an efficient means for converting the polarization state ofreturning rays with respect to their incoming state, in the same manneras was accomplished in FIGS. 4A and 4B. A preferred arrangement toaccomplish this is shown schematically in FIG. 24 for a single input ray340 exiting the projection lens 20. For the preferred process to operateefficiently, a polarization converting layer 342 must act to preventincoming ray 344 from passing through selective reflecting layer 346,while still converting returning ray 348 to the polarization state thatwill reflect from a selective reflecting window layer 350 covering holediameter 352. Consequently, the returning ray 348 is substantially inthe orthogonal state to the ray 344. One way this can be accomplished isby using the polarization converting layer 342 which changes thepolarization of the incoming ray 344 upon passing therethrough, and thenadvancing it (rather than reversing it) in polarization state uponpassing back out through the converting layer 342. Symmetry argumentsgenerally mitigate against such behavior in linear crystalline material.Certain nonlinear or resonant materials are known to show suchcumulative bi-directional effects, e.g., gain in a laser media, andwould be expected to accumulate phase change bi-directionally as well.Nonlinear and resonant effects are, however, typically very wavelengthsensitive, which is not a preferred characteristic for the present imagedisplay applications.

In another embodiment shown in FIG. 25 the same functional result can beachieved as described for FIG. 24 but without need for such anon-standard polarization converting means as the layer 342. A reflector354 closest to the projection lens 20 includes a transparent substrate356, a window layer 358 of diameter equal to the projection lens exitpupil is centered on the optic axis 100, and a polarization-selectivematerial that passes LHCP state input ray, such as the input ray 340,and reflects all rays of the orthogonal polarization state RHCP. Thispolarization selective material is laminated or attached to transparentsubstrate 356, and also included is a metal or metal-likepolarization-changing, reflecting annulus 360 such as wide band mirrorlayer 120 used in FIG. 1A and FIG. 4A, with a hole of diameter, a, alsocentered on the optic axis 100. A reciprocating output reflector 362includes an outer quarter-wave retardation film layer 364, with a hole366 of diameter a/2, centered on the optic axis 100, a metal ormetal-like polarization changing reflecting layer 368 of diameter a/2,also centered on the optic axis 100, a polarization-selective layer 370arranged to reflect P2 and pass P1, and a transparent substrate 372. TheLHCP input ray 340 passes through the window layer 358 as LHCP ray 373,which on retro-reflection at base reflecting layer 368, converts to RHCPray 374. The polarization-selective window layer 358 (such as the Merckmaterial) thus splits unpolarized and polarized light into (1) areflected beam of the RHCP ray 374 and (2) an equally intense beam ofthe transmitted LHCP ray 373. Further, when the embodiment is providedwith RHCP input, pure reflection of the RHCP input occurs. When widerangle input ray 376 passes through the window layer 358 as ray 378, thisray's trajectory just misses the base reflecting layer 368 and passesthrough layer 364, converting from LHCP to P2 at polarization selectivereflecting layer 370, reflecting backwards and converting back to LHCPduring the return path through the layer 364, and emerging as LHCP ray380. When the LHCP 380 strikes the reflector 354, it just misses thewindow layer 358 and reflects off the polarization reflecting annulus360 as RHCP ray 382. When the RHCP ray 382 passes through the layer 364and converts from RHCP to P1, it passes through the polarizationselective reflecting layer 370 as an output ray.

While the invention of FIG. 25 prevents reflected rays from retuning tothe projection lens 20, the method leaves a dark spot or gap 384 in thecenter of the projected image of diameter a (0.1″ in the aboveexamples). Eliminating the spot's visibility requires an efficient andreasonably thin means for displacing all output rays on the periphery ofthis dark spot 384 towards the image center on the optic axis 100. Themaximum displacement for any ray, in this example, is 0.05″ or 1.27 mm.

In an embodiment shown in FIG. 26 beam displacement is performed whereintwo angle transforming films 386 and 388 are separated by either an airor dielectric gap 390. The first film 386 transforms input light 387into a fixed oblique angle that traverses the gap 390 at an angledirected towards the optic axis 100. The second film 388, preferably areciprocal of the first, reverses the process, and converts output ray392 to that of its original inclination. The gap 390 needed between thetwo angle-changing films 386 and 388, for an angular change of ψ degrees(referenced to air or dielectric as appropriate) and a displacement ofa/2, is a/2tanψ. It follows that the same method can be applied as abeam expander just by reversing the direction of input. One possibleembodiment is shown in FIG. 27, for the case of two prismatic films 394and 396. This is illustrated for a case where the same basic prisms 398are used in each of the two prismatic films 394 and 396, but there aremany other embodiments, depending on the application, where prism design(angle and spacing) can be varied. One reason for varying the prismangle is to vary the amount of beam displacement, as for example, fromouter edge of the projected image to inner core, and another reason isto prevent Moire interferences. When identical forms of the base prisms398 are used, the associated beam displacement effected causes the outeredges of the projected image to shrink inside the outer edge of theprimary conicoid, thereby eliminating the possibility of achieving atruly “borderless” image on the projection screen 26. The diagonal ofthe projected image is less than the diagonal of the primary conicoid bytwice the displacement applied. Varying the beam displacement linearlyfrom zero displacement at the outer edge, to the maximum displacement atthe inner edge, maintains the full image edge-to-edge across theprojection screen 26. Any associated image distortion can be compensatedfor electronically.

In the present case, prism angle, α, is 30 degrees, and while thismethod works in some applications for linear prism arrays (groovedfilms), the present application to projection display images with acircular buffer zone assumes that the groove profiles shown represent atwo-dimensional cut through an element with grooved rings, as shown inFIG. 28. Input light, as represented by input ray 400, preferablyapplied at normal or near normal incidence, refracts through first filmelement 402, passing sequentially through its substrate or the baseprismatic film 394, and into the base prism 398 itself (also see FIG.27). The prisms 398 are preferably right angle prisms as shown. Theinput ray 400 exits from the prism's hypotenuse face into air astransmitted ray 404 at an oblique angle β to optic axis 100, that inthis case is 18.6 degrees. For a 1.25 mm displacement, the gapthickness, g, in air is about 3.6 mm, which is not at all unreasonable.The transmitted ray 404 refracts into the base prismatic film 394 ofsecond film element 406 as ray 408, propagates through prism and exitsthrough the prism's hypotenuse face into air as output ray 411 at anangle arranged to be at normal or near normal incidence. This assemblycan be located, as shown, just after the reflector 354 of FIG. 25 behindthe projection screen 26. In principal the Fresnel lens 110, ifnecessary, can be placed either before the first film element 404 orimmediately after the second film element 406. The operative criteria isthat the ray passing through the center of the image at the SLM 14 andprojected by the projection lens 20 preferably arrives at the projectionscreen 26 heading along the optic axis 100 and such that its path lengthfrom SLM 14 to the projection screen 26 matches the focal length of theprojection lens 20.

The length of the base prism 398 is typically 30 to 50 microns, soMoire-type interferences with the system's Fresnel lens 110 can besuppressed. As Moire interferences (visible fringes) are possible bycompetitions between the first and second film elements 402 and 406, itcan be necessary either to vary the prism lengths randomly within eachof the film elements 402 and/or 406, or to choose two sufficientlydifferent prism spacings.

Volume holographic films, such as those manufactured by PolaroidCorporation, diffractive (or binary) optic elements, surface diffractiongratings and gradient index films are among the other mechanisms forangle changing that can each be arranged to work in substantially thesame manner as shown in FIG. 26. Moreover, it is possible to combine twoor more different types of angle-changing elements.

In generalized form, the polarization-dependent folded projection screensystem inventions, such as for example as shown in FIGS. 1A, 7-10 and11-28, each consist of a prepolarized source 12 (or sources), a wideband polarization-selective reflector, a wide bandpolarization-converting reflector, the Fresnel lens 110 and theprojection screen 26, as shown in FIGS. 29-31. In the form of theinvention shown in FIG. 1A and FIGS. 7-10, prepolarized source rays 412as shown in FIG. 29 are selectively transmitted through selectivereflector 414, are processed and returned by a converting reflector 416and then are selectively reflected towards the Fresnel lens 110 and theprojection screen 26. In the form of the inventions of FIGS. 11 and 12,the prepolarized source rays 418 strike a re-directing reflector 420,are directed through a first selective reflector 422 to apolarization-converting reflective element 424, and returned to thefirst selective reflector 422 to be selectively reflected towards theFresnel lens 110 and the screen 26, as in FIG. 30. The form of theinvention of FIG. 13 is also represented by FIG. 30, with the exceptionafter the rays pass through the first selective reflector 422 and theystrike a second selective reflector instead of the re-directingreflector 420, and are otherwise re-directed towards the Fresnel lens110 and the screen 26. In the manner of the inventions of FIGS. 14-20,for the embodiment of FIG. 31 pre-polarized source rays 426 strike andare redirected by a selective reflector 428 towards another convertingreflector 430, and then redirected back through the selective reflector428 towards the Fresnel lens 110 and the screen 26. In each form, any ofthe reflecting elements can be given optical power by virtue of theirsurface shape or by the incorporation of shaped refractive components,or both.

There is one particular embodiment of polarization-selective,image-folding system embodiments when the method of optical powerbecomes particularly important. This class, illustrated in cross-sectionin FIG. 32, is an improvement or extension on the inventions of FIGS.14-20 and their generalized form of FIG. 31 and can include the methodsof FIGS. 21-24. The coaxially curved and preferablyrotationally-symmetric reflecting elements of FIG. 32 are furtherillustrated in three dimensions in FIG. 33 for a circular output, and inFIG. 34 as truncated for the standard 4:3 viewing aspect ratio common toU.S. television. In this variation, considering the profile of FIG. 32,pre-polarized light such as ray 450 from the projection lens 20 isdirected through a small physical hole, or window 434, as in FIGS. 24and 25 in a curved, (rather than flat), polarization converting andreflecting element 436. As before, the window 434 is sized to match thediameter of projection lens exit pupil 438. The curved polarizationconverting element 436 is formed symmetrically about the optic axis 100(and axis of symmetry for this embodiment) in the shape of a primaryconicoid, which faces the convex surface of a smaller coaxially alignedsecondary conicoid 440. The primary conicoid shape of the convertingelement 436 is preferably a paraboloid (or a hyperboloid) whose frontfocus 442 resides on (or near) the back surface of the projection screen26 and whose vertex point 446 resides on the center of the projectionlens exit pupil 438. The secondary conicoid 440 is preferably ahyperboloid (or an oblate ellipsoid), one focal point of which resideson the primary conicoid front focus 442, and the other focal point whichresides on the primary conicoid's vertex point 448 or 442. The secondaryconicoid is composed of the same elements previously described in FIG.25, sequentially from the right to left in the figures, an opaquereflector element 368, a properly oriented polarization-converting layer490, a properly oriented polarization-selective reflecting layer 498,and a transparent support substrate 416. The axis of symmetry common tothe two coaxial conicoids is the system's optic axis 100. Incoming lightrays 450 pass through the primary conicoid, polarization convertingelement 436, strike the secondary conicoid 440, and are reflected backeither by the opaque reflector 368, or by the action of thepolarization-selective layer 498, towards the interior or concavesurface of the metalized interior surface layer of the convertingelement 436, whereupon they are reflected back towards the secondaryconicoid 440, and outwards to the projection screen 26. Thisreciprocating design operates as if input rays 451 striking thesecondary conicoid 440 actually emanated from the common focal point(the front focus 442). Reflected ray 452 is directed along path 454, aline connecting the common focal point (the first focus 442) with thepoint on the secondary conicoid 440 where the input ray 451 isreflected. Because of this, these reflected rays 452 are subsequentlyredirected by the primary conicoid polarization converting element 436in a predictable manner. For example, when the converting element 436 isa paraboloid, output rays 456 exit in the well-collimated mannercharacteristic of a paraboloid.

A preferred form of the above described conicoidal structure is shown inFIG. 35 for a simple paraboloid (i.e., a primary conicoid 458) ofdiameter D with the vertex point 448 and whose focal point (the frontfocus 442) is at D/4 from the system origin which ordinarily is theparabaloidal vertex point. A simple hyperboloid (the secondary conicoid440) has its front focus 442 at D/4, having a point 460 on itsreflecting surface with coordinates (D/3.465, D/4) each referenced tothe system origin. In this case, collimated output rays 462 aredelivered across the entire output aperture of diameter D, and thelimiting cabinet depth, t, is D/4. This configuration, which producescollimated light, eliminates the need for the corrective Fresnel lens110, and can be placed in contact with the projection lens 20. Thesource rays are input through a physical hole (such as the hole 322 inFIG. 20) and some rays are lost by low angle return reflections, thepresence of the hole 322 will cause a dark spot on the projection screen26 at its center. This dark spot can be collapsed by adding the firstand second film elements 402 and 406 described in FIG. 27.

Another preferred embodiment is shown in FIG. 37 having two coaxialhyperboloids, a primary polarization converting element 436 (consistentwith FIG. 38) having a surface point 466 at (D/4, D) and foci atcoordinate point 468 (D/4, 0) and point 470 (minus D, 0), and a smallerhyperboloid secondary element 472 having a surface point at (D/4,D/3.47) and foci at the coordinate point 468 (D/4, 0) and the point 470(0, 0). In this case, the system's +/−35 degree extreme rays 476 and 478are arranged to exit the optical system 10 at 35 degrees, whereascentral rays 480 exit parallel or nearly-parallel to the optic axis 100.In this case, output rays 482 appear to emanate from the primarypolarization converting element 464 at rear focus 484 on the optic axis100 at point minus D. Because of this divergence, the Fresnel lens 110is needed to apply directional correction. This variation increasescompactness by nearly 50% over the embodiment of FIG. 32, with aresulting cabinet depth, t, of D/5.9.

A magnified view of the previous example is given in FIG. 38, to furtherillustrate the behavior of low angle rays. The ray behaviors in FIG. 38are substantially the same as in FIGS. 20-27, except for the effect ofcurved rather than planar reflecting elements. In one variation, allinput rays 486 exiting the projection lens 20 are left hand circularlypolarized (LHCP). In the nomenclature of FIG. 32, one of the centralrays 480 passes through window 488 heading right to left towards thesmaller secondary element 440. On reaching this secondary element 440,the input ray 480 passes through converter layer 490, and converts thelight from LHCP to linear polarization P2. Linearly polarized, the inputray 480B is reflected by selective-reflecting layer 492 back through theconverting layer 490, emerging in the direction of the curvedpolarization converting element 436 as LHCP ray 494. When the LHCP ray494 strikes front surface layer 496 of the converting element 436, it isconverted from LHCP to RHCP and redirected back towards the secondaryconicoid 440 as the LHCP ray 494. On reaching the secondary conicoid440, the LHCP ray 494 passes through the converting layer 490, becomeslinearly polarized as P1, and transmits efficiently through selectivereflecting layer 498 as the output ray 456.

In one of several possible arrangements of output elements, thedirection of the output ray 456 in FIG. 38 is first corrected by itspassage through the Fresnel lens 110 and then by passage through a beamdisplacing element 500 (such as has been described in FIGS. 26 and 27)prior to final passage through the projection screen 26. The beamdisplacing element 500 displaces the output ray 456 a pre-designedamount towards the optic axis 100, effectively filling in the regioncontaining no image information. Alternatively, the effect of thedisplacing element 500 can be effectuated if either by making a tiltcorrection to the polarization converting element 436, as if hinged orpivoted at a point, such as at point 502 or point 504, or by an ogivecorrection (described hereinafter) to the converting element 436. Thedifference between these latter two beam displacement methods is thathinging or pivoting is applied to the upper half and lower half of theconicoidal polarization converting element 436, as in FIGS. 33, 34 and38. Ogiving is a tilt performed in a profile plane that is then revolvedabout the axis of symmetry so it has effect in all other such profileplanes. An ogive surface is one which is generated by the rotation aboutan axis of symmetrical curves lying in a plane so that when segments ofthe curves that are above and below the axis intersect on the axis thetangents to the curves at that point make a non-zero angle with eachother. The name is derived from the architectural description of aparticular type of cathedral arch. In the hinging method all rays abovea horizontal stripe of the buffer zone 148 formed by and on the SLM 14,are each diverted upwards or downwards from their otherwise idealdirections by the deliberate angle of tilt of the polarizationconverting element 436. Accordingly, all rays from the lower-most edgeof the upper image portion 86 arrive at the center of the image plane onthe back surface of the projection screen 26, tilted downwards; andthose rays from the corresponding upper-most edge of the lower imageportion 88, arrive tilted upwards. Despite such slight angular changesat the projection screen 26, a complete image is reconstructed on theprojection screen 26, with no evidence of the once empty “black” stripebetween upper and lower image portions 86 and 88. The ogiving effectoperates the same way, except that the region of black rays (the bufferzone 148) on the SLM 14 is made circular about the SLM's center, ratherthan a horizontal band.

In either case, all light such as rays 506 in FIG. 38 will deviate fromtheir preferred directions by the angle of tilt. The only practicalconsequence of this correction is a slight image shape-error known askeystoning. One method of effecting a keystone correction involvescompensating for the tilting (or ogiving) of the polarization convertingelement 436 by deliberately reprogramming the electronic image pixellocations in the SLM 14, to anticipate not only the “black” pixellocations, but also the predictable spatial effect of keystonedistortion. In this latter method, instead of arranging the image pixelsin a standard rectangular array, the pixels are arranged in the reversekeystone of the distortion anticipated, so that when the actualdistortion occurs, the “distorted” output image at the projection screen26 will be a rectangle of the correct aspect ratio rather than akeystone figure.

With the primary conicoidal converting and re-directing element 436 andthe secondary conicoidal polarization converting and selecting conicoidelement 440 of FIG. 38 taken as, for example, the paraboloidal primaryconicoid 458 and the hyperboloid secondary conicoid 440, as in FIG. 35,the beam displacement method of FIGS. 26 and 27 is applicable without aseparate Fresnel lens element 110, as in FIG. 36. The method of FIGS. 26and 27 can be thought of, in this case, as the use of two reciprocatingFresnel lenses so disposed as to effect the described beam displacement.When the primary converting element 436 and the secondary conicoidelement 472 are both hyperboloids, however, as in the example of FIGS.37 and 38, some additional means such as the Fresnel lens 110 should beapplied first, to “pre-collimate” the divergent rays prior to their usewith the beam displacing element 500. Fresnel lens correction is alsoindicated in this case, in conjunction with either the methods ofhinging/pivoting or ogiving.

The shape of the primary polarization converting element 436 and thesecondary conicoid 440, whether paraboloid or hyperboloid, can befurther modified by (1) incorporating aspherizing terms in the shape (2)splitting the shape into toric sections, each optimized with respect toconicoidal polynomial coefficients, and (3) by having a radially varyingcurvature. A variety of other useful forms and variations, including theincorporation of refractive elements, will be described hereinafter.

The terminology “conicoid” optical element derives from the variousplane sections that can be made in a three-dimensional cone, as shown inFIG. 39. The two dimensional boundary functions so formed by theintersection planes are symmetric polynomials and, when rotated abouttheir axis of symmetry, form the associated conicoids. Plane A in FIG.39 generates a circle, which when rotated produces a sphere or spheroid.Cut in half, this element is a hemisphere, and when rotated is ahemispheroid. Plane B in FIG. 39 cuts through the cone at an angle andforms two parabola sections, either of which when rotated becomes aparaboloid. The size of the paraboloid depends on the location of thecut. Other plane intersections, such as C in FIG. 39 and D in FIG. 39,form families of ellipses (ellipsoids) and hyperbolae hyperboloids),each of whose eccentricity (shape anisotropy) depends on the cut angle.A conicoid is represented mathematically as a polynomial function in zand radial dimension H(x,y) as:$z = {\frac{{cH}^{2}}{\left( {1 + q} \right)} + {aH}^{4} + {bH}^{6} + {cH}^{8} + {dH}^{10}}$

where q²=1−(K+1)ρ², H², H²=x²+y² and a, b, c and d are the aspherizingterms.

When k=0 the function returns a spheroid. When k is negative between 0and minus 1, the function creates an ellipsoid; between minus 1 andinfinity, a hyperboloid. When k=minus 1, the function creates aparaboloid. When k is positive and greater than 0, the function createsan oblate spheroid.

The principal advantage of using reciprocating conicoid's over thereciprocating planes of FIGS. 19 and 20 is cabinet compactness. Thereciprocating hyperboloids of FIG. 37 fit within a cabinet depth, t, ofD/5.9, whereas the shallowest cabinet depth, t, possible withreciprocating planes is D/4.8 for parallel planes and D/5 for tiltedplanes. Only when some optical power (e.g. reflector curvature) wasadded as in FIG. 20 can this level of depth reduction be approached.Applied to the example of a 50″ screen diagonal, cabinet depth, t, canbe reduced by as much as 2.5″ to 8.5″ using optical power, as opposed to10″-11″ when not.

In the preferred embodiment shown in FIG. 32 one can apply the abovereciprocating conicoid method efficiently and without visible imageghosting or intensity non-uniformity by requiring that thepolarization-selective reflecting layer 498 and polarization-convertinglayer 492 be attached in a particular way to the curved surface of thesecondary conicoid 440. This attachment should maintain proper alignmentbetween the preferred orientations in the two layers 498 and 490 and thedirection of polarization for the light rays. Since the input light rays451 are preferably circularly polarized (LHCP), only the orientation ofthe selective polarizer is of concern. This polarized material (such asthe 3M product referenced hereinbefore) is produced in flat sheetshaving a preferred orientation or direction that should be held parallelto the direction of light polarization for maximum transmission, andperpendicular to it for maximum reflectivity, as shown, for example, inFIG. 40, which depicts a typical sheet of such film. This can be atnormal incidence as shown, or the reflecting layer 498 can be rotatedabout axis 508. When the alignment between the layer 498 and the lightis not perfect, as might be the case when a flat film is made to conformto a curved surface, both transmitted and reflected beam components areintroduced, as shown in FIG. 41. The problem is not due to thecylindrical curvature, as shown in FIG. 42, but rather the deformationof the preferred directions when a flat sheet is mapped onto a sphericalcurve, as illustrated in FIG. 41 for P2 (s-polarized) rays 510 inperfect alignment and a similar ray 512 which is mis-aligned. Theimplication of this behavior is that for the incoming ray 512 in FIG.41, rather than being substantially redirected as s-polarized ray 514,some unwanted light rays 516 will be transmitted in polarization stateP1 and P2. These light rays 516 will be misplaced spatially within theimage, and a ghost image will result. The steeper curvature of thesecondary conicoid 440, the more pronounced this effect will becomenearer to its edges.

Since the selective reflecting layer 498 is made in flat sheets, theiradaptation to curved surfaces needs to be done carefully. If cut andlaminated to conform to the curved surface, it is possible that thefilm's orientation vector will point differently in different regions ofthe curved surface, as shown in FIG. 41. The cross-sectional cut made onthe optic axis 100 (see FIG. 42) shows that all alignment vectors arewell-aligned with the light's polarization vector, for every angle ofincidence within the cross-sectional plane. Incident rays headingtowards the rim regions of the curved surface, however, such as point bin FIG. 41, can be mis-aligned with the film's direction vector.

Referring to the relationships shown in FIG. 44, the reflection andtransmission properties of the 3M-type selective reflector film 520 aredescribed in FIG. 45, for measurements made with a polarized HeNe laser.Curve A in FIG. 45 refers to the reflected ray 528 in FIG. 44 for thecase when the angle of incidence of ray 518 is 45 degrees. Curve Brefers to the transmitted ray 530 in FIG. 44 for the same angle ofincidence. CurveC, however, refers to the transmitted ray 530 for thecase where the incident light is normal to the film plane. Incidentlight 518 is taken to be in the x-z plane and impinging on the film'sx-y plane initially at a 45 degree angle. The direction of polarizationis shown in FIG. 44 as being 524 for each of the incident 518, reflected528 and transmitted 530 ray components. Light intensity (reflected ortransmitted) was obtained as a function of the angle made between apreferred orientation direction vector 522 of the film 520 in FIG. 40and the x axis. The film orientation shown in FIG. 44 is 0 degrees.Polarization direction vector 524 is maintained parallel to the y axis.The film orientation angles are changed by rotation about optic axis100, also the z axis. FIG. 45 shows that when the film orientationvector 522 in FIG. 44 and the polarization direction vector 524 also inFIG. 44 are orthogonal (film orientation 0 degrees), essentially all theincident light ray 518 is reflected as ray 528, less any absorption andscattering losses in the film 520, as in CurveA. Also shown in FIG. 45,for the same orientation, practically no incident light is transmittedas ray 530 during this condition as in Curve B. FIG. 45 shows only aminor change in transmission when the incidence angle, previously 45degrees, is reduced to normal incidence or 0 degrees. Polarizationmeasurements were also made to verify the polarization state, and nopolarization conversion was observed. Therefore, the reflected light andtransmitted light polarizations were identical to the incidentpolarization.

The experimental data of FIG. 45 shows that while film orientation is animportant factor over large orientation changes, the performance isrelatively insensitive to moderate orientation changes over at least therange designated as 471. The data associated with 0 degrees is oneexample. There is no measurable performance change within a 10 degreemis-alignment, and less than 10% undesired transmission within a 20degree mis-alignment. Thus, provided the secondary conicoid 472 (thehyperboloid) as shown in FIG. 37 is not made too deep, it is possible tocut a flat sheet of material so that it will conform to the curvedsurface, both with a minimum number of boundaries or seams and withorientational mis-alignments held within this range.

One way to accomplish this preferred alignment between the polarizationof the incoming light rays and the 3M-type film 520 applied to this typeof slowly or weakly curving surface is to form the secondary conicoid472 as a series of segments that can be, for example, circumferentialrings 521 or radial facets 523 as shown in FIGS. 46-7 and 48-9respectively, and then apply the properly oriented and cut film pieces521 or 523 conforming to each region, as demonstrated in FIGS. 46 and48. If the curvature in any given region is arranged to be slight, theinitially flat though compliant plastic film pieces can be made toconform to the curvature without significant shape error, either byadhesive strength alone or with the slight additional stretchingdeformation that would be applied to the film substrate with thecombination of heat and pressure, as in a die-press. Performanceirregularities at the film boundaries can be minimized by precisecutting as with a steel-ruled (zero-clearance) die cut, and amechanically-precise application fixture.

Since the Merck-type circular polarization selective reflecting materialdescribed hereinbefore, is not sensitive to such in-plane angularorientations, its use on the secondary conicoid 440 as the reflectingelement 498, as in FIG. 32, can be preferable to the 3M-type material.In this case however, a half-wave rather than quarter-wave retardationfilm is used for the polarization converting layer 490 as in FIG. 32.

Using the Merck-Type selective-reflecting material 498 in place of the3M-type, as in FIG. 32 for example, the incoming LHCP ray 451 willconvert to RHCP on passing through half-wave converting layer 490, andas such would be reflected by the Merck-type material. After a secondpass through the half-wave polarization converting layer 490, the ray494 would emerge as LHCP, which would convert to RHCP as before, onreflection at the polarization converting element 436 in FIG. 32.Whenever this LHCP ray 494 is redirected back to the secondary conicoidelement 440, it will be transmitted rather than be reflected by theselective reflecting layer 498, because the incoming RHCP ray will beconverted to the transmissive LHCP state by passage through thehalf-wave layer 490.

A most preferred way to assure perfect alignment between the light ray'splane of polarization and either 3M-type or Merck-type polarizationselective reflecting material is to degenerate the conicoidal reflectorsof FIGS. 32-38 to a curved form of the primary (polarization convertingand reflecting) element 436 and a reciprocating secondary reflectorelement composed of a flat (or weakly curved, or a composite of flat andweakly curved) polarization-selective reflecting plane that is combinedwith an associated refractive element that applies the additional amountof optical power needed. This approach avoids the need for thecomplicated film orientation and attachment processes described above.The basic concept is illustrated in FIG. 50 for a concavely-shapedprimary reflector 534, which can also include provisions forpolarization conversion as above, a light inlet hole 536 correspondingto the pupil diameter, a pre-polarized light source 538 supplying eitherlinear or circular polarization, a first refractive element 540, a flatselective reflecting plane 542 and a front refractive element 544. Asshown in phantom in FIG. 50, the embodiments of elements 540, 542 and544 can be replaced by elements 540′, a weakly-curved 542′, and element544′. There are three basic forms of this variation for plane selectivereflectors 554 as shown in FIGS. 51-53: a curved primary conicoidconverting element 534 and a composite secondary element 548 composed of(i) a composite lens 550 with air-gap 552, a polarization selectivereflector 554, a quarter-wave converting element 556 and a circularlypolarized image source 546 (FIG. 51); (i) the polarization selectivereflector 554, the quarter-wave converting element 556, a composite lens562 (with weak center section 564), and the circularly polarized lightsource 546 (FIG. 52), and (iii) the composite lens 550, the polarizationselective reflector 554 and converting element 556 and the compositelens 562 (FIG. 53). Many other related variations are possible when thepolarization selective reflector 554 is deliberately curved over itsentire surface, or only in certain sections. In these cases, the powerof the refractive elements can be weakened proportionally. Moreover, thecurvature of the element 554 can be used as a correction on the designof the composite refractive elements.

In the illustrative design of FIG. 54, primary conicoid 566 is analogousto the structure in FIG. 32, except it is now a very shallow and mildlyconvex paraboloid surface with a focal point 568 shown and vertex 570 onthe optic axis 100 at minus D/0.267 and D/20, respectively. Areciprocating secondary reflector element 572 is a composite of apositive lens 574 and a negative lens 576, shown appearing net negativefor the central portion of incoming angular rays and its retro-reflectedcomponents, and net positive for the higher angle retro-reflectedcomponents. The outer surface of this composite lens 574, 576 is, forexample, a hyperboloid with foci at coordinate points (D/4, 0) and (0,0) and point (D/5, D/2) on the surface. The interior (negative) portionof the composite lens 612, 614 is, for example, also a hyperboloid withfoci at coordinate points (D/5, 0) and (D/20, 0) and point (D/4, D/2.5)on the surface. In addition, proper adjustment of the aspherizing termsof one or more of these conicoidal surfaces is conducted so that theconditions for sharpest focus are achieved at the projection screen 26.As one example, adding aspherizing terms to the hyperboloidal surfacefunction of the interior portion of the lens 576 described above can beaccomplished so that the effect of those terms is to change the slope oftrailing portion 578 of the function more significantly than interiorportion 580. By this means, higher angle ray trajectories, such astrajectories 582, will be affected differently than lower angle raytrajectories 584 which will be more heavily influenced by the interiorportion 580. This adjustment compensates for the fact that lower angleray trajectories make three passes through the interior portion 580 ofthe negative lens 576, whereas the higher angle trajectories 582 makeonly two passes versus three passes. Because of the finite range ofangles around each principal ray, the sharp transition between the netnegative lens portion and the net positive lens portion can result in ablurred image for the corresponding radial transition region, whichmight appear as a thin ring visible to the viewer on the projectionscreen 26. This thin ring corresponds to the angular width of thenegative-to-positive lens transition region. Accordingly, and as onemeans of avoiding this potential artifact, the associated transitionregion can be significantly reduced by applying the same closuretechniques developed earlier for the elimination of the central hole,see FIGS. 21-28. These closure techniques involved the electronicprogramming of the SLM 14 so as to relocate any image information withinthe affected spatial range elsewhere within the SLM's active region, andarranging all image pixels so that a complete and well organized imageresults upon the closure of the affected or “black-ray” spatial regions.Previously, such a region corresponded to the in-coming beam's centralcore. Adding an additional region, such as the composite lens'transition ring, can be implemented at the same time. The Fresnel-likeprismatic beam displacement method of FIGS. 26-28 used to close thebeam's interior core can be used equally successfully to close a radialring

Illustrative LHCP ray 586 in FIG. 54 passes right to left through thepupil-sized window 588 in the primary conicoid 566 heading towards thepositive lens 574. Upon arriving at the lens 574, the ray 586 refractsjust slightly through refractive media 590, then refracts downward andout through the surface of the negative lens 576 upwards into air, whileheading obliquely towards a sequential polarization converting layer 592and selective reflecting layer 594 of planar element 596. The LHCP ray586 thus converts to P2 on passing through a quarter-wave form of thepolarization converting layer 592, reflects off the plane surface of anunderlying 3M-type of the selective reflecting layer 594 and then backthrough the converting layer 592 towards the negative lens 576 andpositive lens 574 and the interior reflecting surface of the primaryconicoid 566 as the higher angle trajectory LHCP ray 582. On strikingthe primary conicoid element 566, the LHCP ray 582 converts to RHCP andheads back towards the composite secondary (the secondary reflectorelement 599) as ray 598. After its composite refraction, the ray 598converts to P1, and then passes outwards, obliquely, through theselectively reflecting layer 594 and encounters the same set ofsequential output elements applicable to the invention of FIGS. 37 and38. Moreover, the beam displacement methods, hinging and ogiving,described above, can be applied equally effectively.

Not only does this arrangement simplify the use of 3M-type of reflectingfilm, but it does so without any compromise in cabinet compactness, allelements fitting within a cabinet depth D/5.8. Although the secondaryconicoid in this variation seems to extend over the entire outputaperture, it does not eliminate the possiblity of the ring-like boundaryedge discussed above, and the methods described above can be used toremove visible artifacts.

One other example of the refractive variation is illustrated in FIG. 55.In this case, a more severely convex paraboloidal primary reflector 600is combined with a polarization-converting layer 602 and 3M-typepolarization-selective reflecting plane layer 604 mated with a truncatedplano-convex positive lens 606 having a hyperboloidal refracting surface608. In this case, the negative power is generated by the parabola, andneutralized at the outer portions of the system by the annular positivelens 610 formed by truncating a plano-convex lens. The effect is adiverging set of output rays that must be managed in the manner of FIG.37. This arrangement fits within a cabinet depth, t, of D/4.7 which isnot quite as compact as the example of FIG. 50 but can be easilyimplemented. Moreover, as the secondary reflector elements of thismethod contain no interior boundary region of the type involved in FIG.50, no electronic and beam-displacement correction techniques are used,other than those related to correcting for the input beam's interiorhole. Yet, preferable designs can apply aspherising terms to the surfaceof the positive lens 610, as well as to the primary reflector 600, so asto produce the most uniform output beam cross-section possible.Tailoring the conicoidal aspherizing terms provides an additional degreeof freedom to correct for non-uniformities.

The diverging set of output rays from the positive lens 610 areconverged towards the optic axis 100 by the Fresnel lens 110 as before.This lens 610 can be planar, as in all previous applications, or curved,to follow the mild curvature of the plano-convex lens, preserving spaceand the boarderless output projection desired. In addition, thehole-hiding method of FIG. 24 is applicable in this case as well, withthe requisite beam displacement achieved through tilting or ogiving theprimary reflector 600, as before, or by inserting a beam displacerbetween the Fresnel lens 110 and the projection screen 26.

Preferable embodiments of each image folding optical system 10 describedabove, depend on utilizing the reliable performance of the wide bandpolarization-selective reflecting film materials. Reliable performance,in turn, depends on two critical polarization-selective filmcharacteristics: (1) the ability of the film to block even trace leakageof the reflected polarization state from the transmitted beam'sorthogonal polarization, and vice versa, and (2) polarizationselectivity at oblique versus normal angles of incidence. In eithercase, however, our main concern reduces to dealing with whether anyfraction of light that should be blocked from transmission, such as, forexample in FIG. 32, the ray 451, actually penetrates through aspremature output rays 612, and otherwise shows up as part of what wouldbe seen as a ghost image. The extent to which leakage is a factor wasevaluated by making actual transmission and reflectivity measurementswith developmental-stage samples of the previously described 3M-typematerial using a polarized HeNe laser. It was found that when alignedfor maximum reflectivity, it is possible that as much as 10% of thereflected light can leak through as transmitted output. Moreover, thepercentage leakage is greatest at lower angles of incidence and isreduced at higher or more grazing angles of incidence.

There is, however, a relatively straightforward polarization-selectivemeans for blocking leakage light from reaching the projection screen 26and creating unacceptable image anomalies. As shown in FIG. 57 a specialclean-up filter element 614 can be added to the optical system 10 at anybeam location after the polarization-selective reflector that is proneto leakage, so as to block (reflect or absorb) the leaking polarizationstate before it contaminates the preferred image on the projectionscreen 26. In FIG. 55, the diverging set of output rays from thepositive lens 610 are converged towards the optic axis 100 by theFresnel lens 110 as before. This positive lens 610 can be planar, as inall previous applications, or curved, to follow the mild curvature ofthe plano-convex lens, preserving space and the borderless outputprojection desired. In addition, the hole-hiding method of FIG. 24 isapplicable in this case as well, with the requisite beam displacementachieved through tilting or ogiving the element 600 as described before,or by inserting a beam displacer in-between the Fresnel lens 110 and theprojection screen 26.

Consequently, in order to block leakage light, one can arrange apolarizer film element in the output beam path such that it is alwayscrossed at 90 degrees with the undesired beam polarization. Two exampledesigns for accomplishing this are illustrated in FIGS. 56 and 57. Thechoice of system location for such design elements depends on the systemembodiment, and whether the embodiment is of the split-image orsingle-image format. For purposes of illustration of the basic conceptof the embodiments, the clean-up filter element 614 or second filterelement 616 is presumed to be located just to the left or right of theprojection screen 26, as in the split image system example of FIG. 1A.

In the split-image methods, for example, of FIGS. 1A, and 7-13, thefilter element 614 in FIG. 56 is composed of two sections of polarizermaterials 618 and 620, each made of either wide band reflectivepolarizer such as the 3M-type film, or preferably, any one of thehighly-transparent and discriminating industry-standard absorbingpolarizer films used commonly in flat-panel LCD displays (such as theNPF series manufactured by Nitto Denko). These two sections areprecisely cut and laminated to a continuous section of transparentsubstrate film 622, with the substrate film 622 facing the projectionscreen 26. Absorptive polarizers are generally preferred over reflectiveones for the polarizer section materials 618 and 620, as absorptioneffectively extinguishes the unwanted rays, whereas on reflection, theunwanted rays can introduce preferentially concentrated regions ofbackground light that might reduce system contrast and uniformity. Lightrays incident on the polarizer section materials 618 and 620, each comefrom either the upper half of the optical system 10, or the lower half,and as such have specifically preferred polarization states. Upper halflight rays, such as rays 624, have already passed through the upper halfof the image of the SLM 14, and are preferably of polarization state P1.Consequently, the clean-up polarizer section material 618, is orientedto maximize the transmission of P1 while minimizing the transmission ofP2 (either by reflectance or absorption). In this manner, andself-consistent with the earlier descriptions, the polarizer sectionmaterial 618 also could be a reflective polarizer material. Thepolarizer section material 618 could preferably be an absorptivepolarizer aligned properly to pass P1. So, any orthogonally polarized P2rays, such as rays 626, that have either been misdirected by the optical10 system or that appear intrinsically as leakage through a reflectivepolarizer, regardless of the reason, and inadvertently strike thepolarizer section material 618, would either be reflected as ray 628 orabsorbed within the polarizer section material 618, but not transmittedto the projection screen 26. Moreover the depth of rejection can besignificant. Absorptive polarizers are far more discriminating than the3M-type reflective polarizers. As a lower bound, however, we can assumethat there has been 10% leakage, and it is being blocked by anappropriately leaky crossed polarizer. In this case, the leakage levelwould drop from 10% to 1%. Using a high-quality absorption polarizer,such as those used in conventional flat-panel LCD displays, thecomparable leakage level is so much lower that if used instead, theprojection screen 26 contamination level would drop to a level that isnegligible in even the most demanding viewing situations. Similarly, thepolarizer section material 620 would be made to reject misdirected raysof polarization P1. Standard anti-reflection coatings can be applied toinput surfaces 627 and output surface 629, to reduce Fresnel losses fromrays such as the rays 624 and 626. Since this cleaning filter element614 can be positioned either in front of or behind the Fresnel lens 110,an embodiment can involve laminating substrate output surface 629directly to the back surface of the Fresnel lens 110, therebyeliminating the possibility of Fresnel losses at that interface.

Another embodiment of the clean up filter element 614 of FIG. 56 isshown in FIG. 57, as second filter element 616 in which a single sectionof the polarizer covers both the upper and lower portions of the element616, and is used as the substrate layer. Proper polarization-selectiveblockage is provided by applying a half-wave converting element 632 overone half of the aperture. One preferable form of the half-wavepolarization-converting element 632 is a wide-band, half-waveretardation film, as described above. In this case, the polarizermaterial 620 has been aligned to pass polarization P2 and reflect/absorbpolarization P1, and the converting element 632 has been aligned so thatpolarization P1 is converted to polarization P2. Accordingly, the upperhalf ray 624 in polarization state P1 is converted to P2 on passingthrough the converting element 632, and then passes through thepolarizer material 620. Note that the converting element 632 has beenapplied only over the top half of the polarizer material 620. Anymisdirected light of polarization P2, such as the ray 626, however,falling on the upper half of the second filter element 616, is convertedto polarization P1 on its passage through the converting element 632,and is therefore blocked by the polarizer material 620. The sameclean-up methods can also be applied to orthogonal states of circularlypolarized light. For example, one continuous quarter-wavepolarization-conversion layer could be added to the input surface 627 ofthe design in FIG. 56. Adding such a layer would convert any state ofcircular polarization to its corresponding state of linear polarizationby virtue of applying a quarter-wave of phase retardation. Once soconverted, the clean up filter 614 performs otherwise as alreadydescribed hereinbefore.

The embodiment of FIG. 57 can also be modified for circular inputpolarizations as well, by adding a continuous sheet of quarter-waveconversion material in between the element 632 and the polarizermaterial 620. In this case, the upper ray 624 is right hand circularlypolarized in FIG. 57, and becomes LHCP on passing through the convertingelement 63, and then sequentially becomes polarization P2 after passingthrough the inserted quarter-wave layer. Converted to P2, the ray 624 isable to pass through the polarizer material 620 as it was for the caseof linearly polarized light.

The projection screen 26 example of FIGS. 56 and 57, while the safestlocation choice for such protection, is perhaps the least efficientchoice for such a protection device. Such a location requires thelargest area coverage and a single device split into two precisesections, and thus can be costly to manufacture. In the case of theoptical systems 10 of FIGS. 1A, and 7-13, these embodiments preferablyuse the location of FIGS. 56 and 57. The optical systems 10 of FIGS.32-38 offer the ability to reduce the filter area, as the clean-upfilter 614 preferably is on the output side of only the secondaryconicoid (440 in FIG. 38).

In another form of the split-image projection system inventions of FIGS.1A, and 7-13, additional elements can be provided to assure that onlylight representative of the upper image region 82 of the SLM 14 in FIG.1A, reaches the upper image portion 86 of the projection screen 26, andcorrespondingly, that only light representative of the lower imageregion 84 of the SLM 14 in FIG. 1A, reaches the lower image portion 88of the projection screen 26. Any trace rays passing through the lowerimage region 84 of the SLM 14 that become part of the upper beam 94, orany trace rays passing through the upper image region 82 of the SLM 14that become part of the lower beam 96, are misdirected and will causeundesirable false images to appear on the projection screen 26. It istherefore desirable to remove all traces of such unwanted polarizationfrom the final image. In addition to the general clean-up filter methoddescribed in FIGS. 56 and 57 above, the buffer zone 148 of FIG. 2 iscreated deliberately within the image of the SLM 14 using the electronicpreprogramming methods that follow in order to separate the upper imageportion 86 from the lower image portion 88 in an unambiguous manner. Itis most likely that some of the rays passing through an infinitesimalboundary region would be misdirected. Rays passing through this smallbut finite buffer zone 148, however, will deliberately not be applied tothe projection screen 26 by the optical system 10, in FIG. 1A. Thesystem 10 will realign the upper and lower image portions 86 and 88 asif the buffer zone 148 did not exist.

In another aspect of the invention, the physical arrangement andelectronic programming of the SLM 14 can be advantageous. One preferredmanipulation of the SLM 14 relates to the polarization-selectivesplit-image methods of the inventions of FIGS. 1A, and 7-13. In thesecases, orthogonal states of prepolarized light pass through the upperand lower image regions 82 and 84 of the SLM 14, as in FIG. 1A. When theSLM 14 is not polarization sensitive, such as is the case with a DigitalMicromirror Device (DMD) or with a polymer dispersed liquid crystal(PDLC) device, no special physical precaution is needed. When the SLM 14is polarization dependent, such as is the case with conventional liquidcrystal devices (LCDs), some minor modification is desirable to assurecompatibility.

Ordinarily, as shown in FIG. 58, input polarizer 634 of an LCD form ofthe SLM 14 assures that only light of one preferred polarization statepasses through the LCD. Bright LCD pixels are then defined by the LCD'saction on the light allowing it to pass through an output polarizer (oranalyzer portion) 636 of the LCD 14. Dark LCD pixels are then defined bythe LCD's action on the light, preventing it from passing through theoutput polarizer 636 of the LCD. The LCD form of the SLM 14 alsocontains an internal alignment layer 638 located on one of the LCD's twoglass plates 649 that has been preconditioned (mechanically) so as toexhibit a preferred alignment direction for the liquid crystal layerthat is related to the orientation of the LCD's input polarizer 634.This preferred alignment is equivalent to establishing a preferreddirection of the plane of input polarization. When input rays 642 and644 from the light source 12 are differentially polarized as in FIG. 62,a conventionally prepared LCD used with this input light could beoptimally aligned internally only in one region. As shown in FIG. 59, toavoid such a mismatch, the LCD 14 can be pre-aligned differently in eachof its upper region 646 and lower region 648. Since the LCD's alignmentlayer 638 is processed automatically during manufacture, and thedevelopment of micro-alignments (multidomains) have become routine,developing two orthogonally aligned LCD regions, such as the regions 646and 648, is not a difficult requirement. Moreover, any LCD whosealignment direction is at 45 degrees to the plane of input polarizationcan be made to operate optimally with two regions of orthogonal inputpolarization.

Whether the LCD's input light 641 is unpolarized, as in FIG. 61 by inputpolarizing elements 634A and 634B or is pre-arranged to be in twoorthogonal states 642 and 644, as in FIG. 62, an attached inputpolarizer 634 is preferably used. If the input polarizer is not neededto polarize input light as in FIG. 62, then it can be added to assurethat no pre-polarized input light of the wrong polarization state isable to leak through, contaminating otherwise purely polarized light.For the embodiment of FIGS. 61 and 62, this input polarizer 634 cannotbe applied across the whole LCD aperture, as is conventionally done, butrather it is preferably applied as two separate and orthogonally-alignedinput polarizer layers 634A and 634B These polarizing elements 634A and634B are applied across the LCD's input aperture as done in FIGS. 59, 60and 61. Steps must be taken, as previously discussed depending on thetype of the LCD 14, so that, despite the bifurcated input polarization,the LCD 14 properly displays a consistent output image. FIG. 61 presumesthe unpolarized light 641 of circular cross-section becomes polarized bythe action of the bifrucated LCD input polarizers of FIGS. 59, 60 or 61.FIG. 62 also presumes a circular input beam, but one that has beenpre-polarized, the upper half in polarization state P1 and the lowerhalf in the orthogonal state P2. The overlap of this circular beamcross-section with the rectangular LCD (or SLM) 14 is shown in FIG. 63.When the pre-polarized input beam is arranged to have a rectangularcross-section, as in FIG. 64, the overlap with the LCD (or SLM) 14 ismuch improved. The polarized output beam of FIG. 64 is then processed bythe action of the polarizing beam-splitter 22, as in FIG. 65, whichproperly sorts the orthogonal polarization states into the two separateoutput beams 94 and 96, one corrsponding to light that was passedthrough the LCD's (or SLM's) upper region 82, and another correspondingto the LCD's (or SLM's) lower image region 84.

One common type of LCD layer 650 (see FIG. 58), can be a super twistednematic (STN), which is normally birefringent in the absence of anapplied voltage 652, V_(a), applied across any or all pixels. When thissufficient voltage 652 is applied, the birefringence (present where anelectric field associated with the voltage exists) drops to zero. TheLCD's internal alignment layer 638 (see FIG. 58) is formed so that theintrinsic birefringence is aligned properly with the plane of inputlight polarization such that, for example, the upper image light ray 642passing through the upper half of the LCD 14 (on passing through the LCDlayer 650), undergoes one half-wave (90 degrees) phase retardation. Theassociated rotation of the plane of polarization for the light ray 642causes, for example, complete blockage by the LCD's output polarizer636, and the alignment, in this case, is made orthogonal to that of theinput polarizer 634. As such, those pixels that do not receive thisapplied voltage will appear black; and those pixels that do receive thevoltage will appear white (or take on the color of any included colorfilter). The reverse operation is also possible. In the illustrativecase, the orthogonally polarized lower image input ray 644 will not givethe same result, unless either the LCD's alignment layer 638 isbifurcated, as described above, and aligned so that the LCD'sbirefringence in the lower half of the device is aligned properly forthe orthogonally polarized light. Alternatively, as seen in FIG. 60 theLCD's output polarizer 636 is bifurcated, and the lower half 636B isrotated with respect to the upper half 701A by the proper amount tocause the same degree of light blockage in the lower half of the deviceas in the upper half of the LCD 14. The LCD 14 can also be of theactive-matrix or TFT type, where the LCD layer 650 is normallytransparent with no phase retardation or optical activity occurring inthe absence of the applied voltage 652 (see FIG. 58). The plane of inputpolarization rotates with the application of the voltage 652 by 90degrees, and a similar situation exists with that of the LCD layer 650.

The level of the voltage 652, V_(a), applied to each of the pixelsmaking up the LCD's image determines whether the pixel appears colored(i.e., white, red, blue, green) or black, by determining the level oflight intensity or brightness measured when considering light from eachindividual pixel. In most cases, one LCD is used for each of the threeprimary colors. In some cases, a single LCD has colored sub-pixels. Ineither case, whether output light from any particular pixel reaches theprojection screen 26, depends on the applied voltage 652 to that pixel.Voltage is conventionally applied to the STN type of the LCD layer 650by a method known as passive matrix addressing through a grid ofelectrode bars on the inside of each of the glass plates 640 (forexample, see FIGS. 59 and 60). These plates 640 apply an electric fieldto any LCD pixel via the voltages at the crossings of the two orthogonalelectrode grids, powered by active electronic devices (chips) located onthe periphery of the LCD's aperture, one per pixel column and one perpixel row. Voltage is conventionally applied to these TFT LCD form ofthe SLM 14 by using the same type chip-driven row and column electrodebars, except the final applied voltage on each pixel is set by means ofan active electronic device (thin film transistor or TFT) located withineach and every pixel, and formed on the inside of one of the glassplates 640. Interconnection is made to each TFT using the row and columnelectrode grid and common (ground) plane located on the inside of theopposing glass plate 640. The incoming image data stream can be thoughtof as a de-multiplexed or sequential stream, where, for example, 8 bitdata defines the intensity of each pixel in the image. This image datais re-multiplexed by the LCD addressing format. The input data is fed tothe chip series (row and column) that holds enough data for one imageframe. Each column and row chip emanates respective voltage waveformsthat are timed properly so that the row and column waveforms interact insuch a way that determines how much voltage is applied at each pixellocation, whether directly to the LCD 14 or first to control asemiconductor switching device located on or within the pixel. Thewaveforms are stored in a look-up table in a controlling semiconductordevice or chip. The desired voltage state for every image pixel locationon the LCD 14 is temporarily stored in the short-term memory provided byeach row and column device. When every pixel has been addressed in thismanner, one image field has been properly established; and the processis repeated in a synchronous manner. For video applications, such afield is established on the order of once every {fraction (1/60)}th of asecond. One video field involves about 500,000 bytes (0.5 MB) of memoryfor SVGA image resolution, and as much as about 1,500,000 bytes (1.5 MB)for the highest image resolutions currently envisioned. To process 500MB of data in {fraction (1/60)}th of a second requires a processingspeed of 30 MHz; 1.5 MB a processing speed of 90 MHz. Accordingly, it isnot difficult to devote a single data processor or content addressablememory device, each including just enough local memory to store a fixeddata transformation algorithm, for the purpose of adjusting the incomingvalues of an image data stream. In this manner, rather than having tophysically rotate the LCD's output polarizer 636 to accommodate theorthogonally polarized light in the lower portion of the LCD 14, we caninstead produce the same “rotation” effect electronically, as isschematically represented in FIG. 66. The LCD 14 of FIG. 58 is addressedby processing the demultiplexed or sequential image pixel data streamassociated with the lower image light 644 sequentially with asemiconductor processing device 656 shown in FIG. 66. This processingdevice 656 contains the permanent data transformation algorithmused, andthe device drivers for each of the LCD's pixel rows and columns 658, toaddress each pixel in the otherwise ordinary manner. The processingdevice 656 would make no correction to any pixel located in the upperhalf of the LCD image, but would adjust every voltage applied to pixelsin the well-organized data stream known to be located in the lower halfof the LCD 14 and do so in accordance with the predicted behavior oforthogonally oriented input light. There are at least two ways this bitstream processing can be done. The processing device 656, including somememory and a hardware multiplier, is preprogrammed so that the voltagemultipliers required for the transformation are stored in memory. Thehardware multiplier is then synchronized with the pixel stream so thatevery incoming pixel voltage is correctly multiplied by itscorresponding transformation value flowing from memory. Yet another wayto make this transformation is to use content addressable memory or amemory map. A counter is initiated when the image pixel stream startsflowing, assigning each pixel location and intensity to a correspondingmemory location. When this data flows into the address port of memory,what flows out will be properly transformed. In either case, handlingSVGA images in this way requires a 30 Mhz processor and 0.5 MB ofmemory—both reasonable possibilities given today's state ofsemiconductor processor technology. As one example of this electronictransformation approach, consider the case when a completely white (orbright) field is desired in both the upper and lower LCD regions. As hasbeen common practice, no voltage would be applied to any TFT pixel,whether in the upper region or lower region, and the maximum amount oflight transmission would result everywhere over the aperture. When thelower portion of the LCD 14 is fed with input light that is orthogonallypolarized with respect to the upper region input rays 642, the lightoutput from the lower region of the LCD 14 would not be maximallytransmitted, but would instead be blocked by the output polarizer 636,which was prealigned to transmit the orthogonally polarized light. Toremedy this, the processing device 656 would be programmed to transformeach of the lower pixel's voltage from zero to the voltage required fora phase shift of 90 degrees. Given a phase shift of 90 degrees, thelower region input rays 644 would have a plane of polarization whichwould become parallel to the upper region input rays 642 and wouldtherefore pass through the LCD's output polarizer 636. Such voltagecorrections can be achieved on a pixel-by-pixel basis for all othervalues of the lower region's input voltage between zero and the valuenecessary for 90 degrees of phase shift.

The same pixel processing methods can be applied, for any form of theSLM 14, to create the deliberate buffer zone 148 between the upper andlower regions 82 and 84 in FIG. 2 and, for example, FIGS. 61-65 or theso-called region 326 of “black rays” associated with the embodiment ofFIG. 20. Despite the conventionally contiguous input data stream for thelower image input rays 644, where one voltage state exists for everypixel in every row in the image frame, the processing device 656 ispreprogrammed to fill the predetermined number of pixel rowscorresponding to the upper image region followed by a preset number ofdummy voltages corresponding to the present number of pixelsrepresenting the preset number of buffer rows prior to sending the pixelvoltages corresponding to the lower portion of the image. The increasednumber of pixels used can be accommodated either by reducing the image'svertical resolution by the width of the buffer zone 148, or byincreasing the number of addressable pixels in the SLM 14. As anexample, suppose the image data is to be in SVGA format (800×600), theSLM's active region has a 0.7″ diagonal, and the desired buffer zone 148only compromises 2.5% of the active region's area. The maximum size ofeach pixel in this case is 17.78 microns square, and the 2.5% bufferzone 148 therefore is 15 rows high by 800 columns wide. Accordingly, the800 column wide upper image region would be made to occupy the first 300rows, starting at the top of the SLM 14, followed by the fifteen rowbuffer zone 148, and finally the remaining 300 rows of the lower imageregion. For this configuration, the total SLM active area would need tobe enlarged to 800×615, either by keeping the same 17.78 micron pixelsize and expanding the SLM's diagonal, or by reducing the pixel size.(Note: As the DMD form of the SLM 14 has a fixed pixel size, and videodisplay resolution standards exist, the preferred way of accommodatingthe increased number of pixels in the buffer zone 148 is to increase thetotal number of pixels available.)

Such SLM programming techniques can also be extended to provide a meansof electronic image alignment fine-tuning on the projection screen 26.We indicated hereinbefore that the invention of FIG. 1A is preferablycarried out to form a seamless re-splicing of the upper and lower imageportions at the projection screen 26. Without being able to adjust therelative locations of the different portions of the split image on theprojection screen 26, the viewer might notice a dividing line betweenthe upper image portion 86 and the lower image portion 88 in, forexample, FIG. 1A. Conventional methods can be introduced to avoid thispotential defect in the image, including preferably adjusting thephysical alignment or tilt of the folding mirror 106 used in theinvention of FIG. 1A. In combination with such methods, the SLM 14 canbe programmed to allow for a final “electronic” correction, appliedafter the best possible mechanical alignment. This can be accomplishedby enlarging or decreasing the width of the buffer zone 148 by one (orpossibly two) row of pixels.

Yet another way in which such SLM programming techniques can be extendedis to provide a fixed electronic means that corrects for intrinsic imageshape distortions such as keystoning. Discussed hereinbefore, keystoningis the image shape distortion that occurs when a central ray 788 in FIG.67 defining the center of the projected image is not maintainedperpendicular to the projection screen 26 and arrives at the focal plane(the projection screen 26) at an oblique angle to the optic axis 100.The basic relationships associated with this effect are shown in FIG.67, and the manifestations with regard to image shape in FIGS. 68 and69. In addition to shape distortion, the tilt of the image plane bothlengthens or shortens the optical path between the image plane and theprojection lens, which so introduces focusing errors. Calculations fortilt angles 660 in FIG. 67 of up to 15 degrees from the optic axis 100indicate only small amounts of shape and path length distortions thatcan be easily corrected, as will be shown. The larger this angle, thegreater the distortions and the larger the need for correction.Correction preferably involves both an electronic means for anticipatingthe effect of the shape distortion that the system will produce and anoptical means for compensating for associated optical path lengthdifferences that defocus the otherwise distorted image shape. The basiccorrective method of electronic programming therefore anticipates theamount of keystoning that any of the above physical projection systemshave been constrained to develop, and then arranges the spatial locationof the image pixels in a structure corresponding to the reverse of thisimage shape deformation. Suppose, as one example, that distorted image662 shown in FIG. 68 is the anticipated output for an originallyrectangular image 664 that would otherwise have filled the projectionscreen 26. The original image, rather than being programmed as a fillypopulated rectangular grid of pixel locations, the SLM 14 would beenlarged, and the pixels arranged as shown in FIG. 69. Rectangle 666corresponds to the originally rectangular active image region, rectangle664 corresponds to a new SLM active region, rectangle 668, to the newactive image pixels, and region 670 to inactive or dark image pixels. Inaddition to the electronic programming means which compensates for theshape deformation, one of two associated optical compensation isdesirable to adjust for the differences in optical path length caused bythe tilted image plane, and the defocusing of the image brought about bysuch path length differences. The defocusing error associated with theoblique tilt angle 660, φ in FIG. 67, can be compensated, either bytilting both the SLM 14 and the projection screen 26, as shownschematically in FIG. 70, or, preferably, by using the simple refractivecorrection plate (wedge) 672 shown for the upper half of the SLM imagein of FIG. 71. The refractive wedge plate 672 operates as shown firstconceptually in FIG. 71 and then optically as in FIG. 85, to move thefocusing point D of rays 803 and 806 from the upper image, to point E.The wedge thickness T in FIG. 85 corresponds to a portion of thecomplete wedge 672 as shown in FIG. 71. The complete correction methodis shown schematically in FIGS. 72 and 73 for application with andwithout, respectively, the corresponding electronic SLM programming forreversing the shape deformation.

In the optical system 10 of, for example, FIGS. 1A, 7-13, 20, 21, 32-38,and 54, particular attention has been paid to all three importantaspects of the projected image, namely the image shape, the sharpness ofthe image and the directionality of the light emerging from theprojection screen 26. The problems of image shape and the steps taken tocorrect the shape have been introduced in terms of the image shapedistortion known as keystoning. The image sharpness and steps taken toensure that a satisfactory level of sharpness is achieved have beendiscussed in terms of optical path length. The directionality of theemerging light at the projection screen 26 is controlled by the use of aFresnel lens 110.

These issues can be described on a more mathematical basis using thespatial relationships defined in FIG. 67. PRQ represents the area to beprojected, the SLM 14, such as an LCD or a DMD, or even a sheet ofmicrofilm, a photographic slide or a transparency. The center of theprojection lens 20 is taken at point O, and the normal position of theprojection screen 26 on which the projected image is to be formed isalong DAE. With the projection screen 26 in the position shown by DAE,the shape of the rectangular image is correct. In this situation, asquare in the plane QRP is reproduced as a square in the plane DAE. If,however, the projection screen 26 is tilted through an angle φ, then theimage on the projection screen 26 has the form shown in FIGS. 67 and 68.In FIG. 67 the following relationships apply:

AB=S1

AC=S2

AD=AE=S

RO=D1

OA =D2

S=(D2)tan(θ)

S=(D2)sin(θ)/cos(θ+φ)

S1=(D2)tan(θ)/[cos(f)−(sin(f)tan(q))]

AD=S=(D2)tan(θ)

S1/S=1.0/[cos(+)−(sin(φ)tan(θ))]

S2/S=1.0/[cos(+)+(sin(φ)tan(θ))]

The fact that S1/S is greater than unity is responsible for theelongation of the upper image portion 86 of the projected area shown inFIG. 68. Correspondingly, the fact that (S2)/S is less than unity givesrise to the compression of the lower image portion 88 of the projectedimage. The horizontal elongation of the upper image portion 86 of theprojected image is also due to the fact that (S1)/S is greater thenunity, while the horizontal shortening in the lower image portion 88 isdue to the fact that (S2)/S is less than unity. The effect of thesefactors is that the shape of the projected image, shown by dotted lines662 in FIG. 68, has the form of the keystone in an architectural arch.Methods for correcting this distortion have been already set forthabove.

In all the folded-optic projection system examples, including those thatfollow, the projection lens 20 is assumed to have a ±35 degree angularrange, θ, which in the vertical (4:3 TV screen) profile, such as that ofFIG. 1A, reduces to ±22.8 degrees, and will be used hereafter. In thisinstance, the implications for several values of the distortion angle,φ, are:

φ=5 degrees; S1/S=1.038; S2/S=0.965

φ=10 degrees; S1/S=1.080; S2/S=0.931

φ=15 degrees; S1/S=1.127; S2/S=0.899

The lateral (or horizontal) magnifications, M1 for the upper imageportion 86, and M2 for the lower image portion 86, take the form:

M1=1.0/(1.0−tan(φ)tan(θ))

M2=1.0/1.0+tan(φ)tan(θ))

φ=5 degrees; M1=1.038; M2=0.965

φ=10 degrees; M1=1.080; M2=0.931

φ=15 degrees; M1=1.127; M2=0.899

These values provide the information needed to predict the shapes of theprojected image in every situation.

As introduced above, electronic methods are applied to correct for imageshape deformations. Corresponding optical methods have been applied torestore sharp focus, and will be considered mathematically below. Inaddition, when dealing with the raster scan of an SLM (LCD or DMD) 14,the packing density of the raster lines becomes important, and must alsobe considered in designing a high-quality projection ssystem.

The restoration of sharp focus can be established, as shownschematically in FIG. 70. The requirement is that the plane of the SLM14, such as an LCD or DMD, also is tilted as shown, so that thecontinuation of the planes of object 792 and image 794 intersect on aline S through the center of the projection lens 20. If themagnification produced by the projection lens 20 is M, and if therespective plane tilt angles are φ₁ and φ₂, then:

tan(φ₁)=(M)tan(φ₂)

The magnifications contemplated in this embodiment are of the order of50× to 70×, so that the tilt of the object plane is quite small. Thisopens up the possibility of establishing a sharp focus by using the(wedge-shaped) refractive correction wedge 672 as shown in FIG. 71. Thelocal thickness W of the wedge 672 is given by the equation (for smallangles of φ₂) by:

W=φ ₂ n/(n−1)

where n is the refractive index of the glass or plastic used in thewedge 672.

We must also assure that there is a proper packing density of rasterlines, PD1, for the upper image portion 86 of the projected image, PD2for the lower image portion 88 of the projection screen 26, and PD, thepacking density in the center of the projected image. Accordingly,

PD 1/PD=cos(φ)/[cos(φ)−sin(φ)tan(φ)]²

PD 2/PD=cos(φ)/[cos(φ)+sin(φ)tan(φ)]²

Whenever PD1/PD is greater than unity, the raster line images will bebroadened out in the upper image portion 86, and narrowed in the lowerimage portion 88. In developing the preferred embodiments of theinventions where a correctable amount of keystone distortion has beenallowed (i.e. with φ up to 15 degrees), care should be taken to includeboth of these factors into account.)

The desired optical path length, D′, as shown in FIG. 84, from theprojection lens 20, for a point on the projection screen 26 reached by aray making an angle θ with the lens optic axis 100 (see FIG. 84) isequal to D/cos(θ). This relationship applies to all the compactfolded-optic projection systems 10, such as for example FIGS. 1A, 7-13,20, 21, 32-38 and 54, where the most preferred goal is typically todevise systems which will have optical path lengths according to thisformula. In some embodiments of this invention, however, it is desirableto depart slightly from this specification of the optical path length.One example is when we choose to accept and then correct for a smallamount of the keystone distortion as above. In this case, when smallamount of keystone distortion is permitted, it is to be corrected by theabove methods, maintaining image sharpness by tilting the SLM 14 objectplane, or preferably by the use of the weak refractive compensatingwedge 672, as in FIGS. 72 and 73.

If the optical system 10 is producing an image magnification M from theSLM 14 to the projection screen 26, and if the optical path lengthinvolved as measured between the projection lens 20 and the projectionscreen 26 shows an error in optical path length, S, this translates intoa focusing error of S/M² in the plane of the SLM 14. Sharp focus wouldbe reestablished, however, if those rays emanating from any region onthe SLM 14 were made to pass through an appropriate thickness ofrefracting material, e.g. the refractive wedge 672 of FIGS. 71-74 and85. If the path length is to be decreased by S, then the additionalthickness preferred of this refractive material is S/M². If, on theother hand, the path length is to be increased by S, then the thicknessof the refractive material would have to be reduced by SM² in therelevant areas. This effect on light rays in the region of the SLM 14 isshown in FIGS. 71-74 and 85. The effect on light rays in the region ofthe projection lens 20 is increased by a factor of M³ over that in theregion of the SLM 14.

Some rays emanating from any given microscopic region on the SLM 14 andtraveling through the correcting wedge 672, are made to travelincrementally longer optical paths than they otherwise would in air, andothers are made to travel incrementally shorter optical paths than theyotherwise would in air, the result being that when all rays pass throughthe folded-optic projection system 10 as above, they arrive at theprojection screen 26 within the smallest possible circle. If the area onthe SLM 14 is equivalent to a pixel element, the area on the projectionscreen 26 formed by the projection of rays from this pixel must notexceed half the magnification of this pixel on the projection screen 26.

This mechanism can be seen in FIG. 85 wherein rays 803 and 806 aredirected along the paths A1-C1-D and A2-C2-D respectively in the absenceof a glass sheet are displaced to A1-B1-E and A2-B2-E by refraction atthe glass or plastic layer interfaces. The image formed by the incomingray 803 and the ray 806, such as those shown, is displaced from D to E.If the glass or plastic layer index is n, and if the thickness is T,then the distance DE is equal to T(n−1)/n. If the optical path error isa function of the image position on the projection screen 26, then thethickness correction at the plane of the SLM 14 (or other image source)has to be adjusted on the wedge 672 near this plane. In order to reduceany optical aberrations, this correcting material should be placed asclose as possible to the SLM 14 plane. In the absence of suchcorrection, a point on the projection screen 26 corresponds to acircular area (a “blur circle”) on the SLM 14 plane. If the lens has anf/#N, then the diameter DM of this circular path is given by theformula:

DM=S/((M ²)(N))

In a specific example, S=5, M=50 and N=2.5, and this gives a value forDM of 0.0008 inches (20 microns). This is compared with the actual pixelsize involved with the SLM 14 that is used. A typical value for thepixel size for an LCD form of the SLM 14 is about 18 microns×18 microns.For a DMD form of the SLM 14, the corresponding size is 16 microns×16microns, with a 1 micron spacing between elements. In order thatinformation is not lost on the projection screen 26, the diameter of theblur circle on the LCD (or DMD) 14 should preferably not be greater thanone half of the pixel size. This shows the need to keep the optical pathvery close to the value predicted by the formula, or failing that, totake corrective measures at or very near to the plane of the LCD or DMD14. If these conditions are not considered, projected images will not beoptimal.

The split-image projection system embodiments of FIGS. 1A and 7-13 eachrequire the beam splitter 22 efficiently divides the orthogonallypre-polarized upper polarized beam 94 and lower polarized beam 96,respectively, passing through the upper and lower image regions 82 and84 of the SLM 14 into two separate beams, one directed ultimatelyupwards toward the upper image portion 86 of the optical system 10 andthe other directed downward toward the lower image portion 88 of theoptical system 10 for cases where the pre-polarized light 24 and 28comes directly from the output of an SLM 14 (see FIG. 74) or from theoutput of the projection lens 20 imaging the SLM 14 as shown in FIG. 75.Upper and lower beam direction elements 674 and 676, respectively, areused so that each output beam 678 and 680, respectively, can be directedat the precise angle expected by the projection system mirrors, such asthe folding reflector mirrors 106 and 108 in FIG. 1A. In addition, upperand lower polarization filters 682 and 684 are used to remove anycontaminating polarization content from each of the upper and loweroutput beams 678 and 680 so as to prevent artifacts visible in theprojected image.

The traditional form of the beam splitter 22 typically uses prismscoated with conventional polarization-diffracting inorganic multi-layerfilm stacks and/or a plurality of glass plates making Brewster's Anglewith the light direction. The more plates in the Brewster stack, themore efficient the beam splitting characteristics, but the less overalllight that is transmitted. Neither of these approaches are preferred,however, for use with the above embodiments because they typicallyoperate too inefficiently over the wide range of wavelengths and widerange of incidence angles involved in commercial forms of the opticalsystem 10. Prior art beam-splitters have not been developed for thesepurposes as can be noted by reference to FIGS. 76-78.

As one example of the preferred embodiments of the inventions considerfirst a prior art beam splitter as shown in FIG. 76. This structure isgenerally unsuitable for use with the inventions described above,because the resulting output beams 686 and 688, while being directed bythe action of elements 690 and 692, are heading in the same direction,rather than opposite directions. The elements 690 and 692 also are usedfor the purpose of beam overlap, rather than to separate the desiredfinal beam location. Moreover, the two output beams 686 and 688 of FIG.76 are arranged to have the same, rather than orthogonal polarizations.Preferred splitter embodiments of the invention are indicated in FIGS.79 and 81-83 and these embodiments arrange for the two output beams 678and 680 from FIG. 74 to travel in opposite directions in a plane that isperpendicular to the input beam direction. More fundamentally, however,the design of FIG. 76 does not produce the output beams 686 and 688having equal optical path lengths, a deficiency that if not correctedwould interfere with the creation of a well-focused image. Thedifference between optical path lengths 1-2-3 and 1-4 in FIG. 76 isapproximately D/n, where n is the refractive index of the prism mediumand D is the height of the entrance aperture.

As another example, consider the prior art beam splitter 694 of FIG. 77.In this case, although there appears to be an upper beam 696 and loweroutput beam 698 that head in opposite directions in a planeperpendicular to the input beam direction, directing elements 700, 702,704, 706 and 708 are employed, as in FIG. 76, to make these beamsadjacent and heading in the same direction. Moreover, convertingelements 709 are employed to make these beams 696 and 698 the same,rather than of orthogonal polarization. In addition, as in FIG. 76,there is an uncorrected difference between the optical path lengths ofthe upper beam 696 and the lower beam 698 that is also equal to D/n.

In a preferred embodiment of the invention, the beam splitter of FIG.79, has been arranged for use in situations like that of FIG. 1A. Thebeam splitter 22 is composed of a 45 degree-45 degree-90 degree (Porro)prism 714 composed to two smaller Porro prisms 710 and 712, refractiveelement 714, two refractive beam directors 716 and 718, and twopolarization filters 720 and 722. In this case, polarization splittinglayer 724 is preferably the same wide band polarization type selectivereflecting materials described hereinabove and referred to aspolarization selective reflectors such as those containing the wide bandselective reflecting polarizer materials 116 or 118 as in for exampleFIG. 1A. These materials enable the full angular extent of input beam726 to be handled as efficiently as possible. Inefficiencies inpolarization splitting can translate into spatial intensity variationsacross the upper output beam 736 and can require additional compensatingelements. The use of wide band materials such as the 3M-type multi-layerdielectric stack film described before, obviates or minimizes the needfor such correction. Reflecting layer 728 is a metal or metal-like film,or in some cases, a total internal reflecting layer. Illustrative inputray 730 of mixed polarization states P1 and P2 is split into two rays bythe beam splitter 22, an upward ray 792 is in polarization state P2 andray 734 heading left-to-right is in the orthogonal polarization state P1polarization. The ray 792 proceeds upwards until it is filtered by thepolarization filter layer 720, preferably by a high-quality absorptionpolarizer oriented to absorb polarization P1 and pass P2. When theoutput beam 736 refracts into air, the tilt of the beam-director 716causes the output beam 736 to point in the direction (or tilt at anangle θ₂) indicated by the embodiment of FIG. 1A, or by the particularprojection system embodiment used. The orthogonally polarized ray 734 isredirected without change in polarization by the reflecting layer 728(which can be either the boundary between the prism 712 and air or areflective material) and passed sequentially through the beam-director718 and the polarization filter 722 as lower output beam 738.

In the preferred embodiment of FIG. 79, it is desirable to control thesize D′ of input face 740 relative to the diameter, a, of the input beam726. Upper beam path 1-2-3 has a length equal to 2D/n. Although theinput beam 726 is drawn as being highly collimated, for clarity andscale, it is actually representative of the bundle of rays that areoutput from the projection lens 20. When the projection lens 20 hasf/2.5 and with an angular range of ±35 degrees in air on the diagonal,the beam angle in the vertical plane is ±22.8 degrees and in therefractive medium, 15 degrees. The actual beam spread in the refractivemedium, when the un-folded beam path is properly represented, FIG. 80,must be taken into account when choosing the size D′ of the beamsplitter 22 that works optionally. The relationship between a and D′ isgiven by:$D^{\prime} = \frac{a}{1 - {4\quad \tan \quad \varphi_{m{\lbrack\quad\rbrack}}}}$

where D′ and a are as previously defined, and indicates that the beamsplitter 22 of FIG. 79 is generally impractical for beam angles largerthan about ±12 degrees in the medium, where D′ would be no greater thanabout 1.5″. Such restrictions can limit use of this beam splitter 22 inthe practice of the above inventions to situations where the projectionlens 20 has a maximum angular range no larger than about ±26 degrees onthe diagonal in air. Use of a more divergent form of the projection lens20 requires using a different class of the beam splitter 22 compared tothat of FIG. 79.

For the splitter 22 to be practical over the full angular range desiredin preferable embodiments of the inventions, such as FIG. 1A, its sizeis governed by an equation where:

1−N tan φ_(m)>0

and, for compactness as defined by element size no larger than 1.5″,where

N tan φ_(m)<5/6

For the case where the beam angle in the medium is ±15, N must be lessthan 3.1. In general, for this to be possible, the beam path from theinput face to the output face through the beam splitter 22 should not begreater than 3D′, which for best results means the value D′.

One example embodiment in FIG. 81 is of a splitter configuration withinput-to-output path length equal to D′. A cube is arranged with fourindividual Porro prisms 742, 744, 746 and 748 and including polarizationfiltering and beam directing elements 752 and 762, and the use of 3M orMerck-type material wide band polarization selective reflecting films,respectively. An example of the tapered wedge type beam director 752 and762 is shown in FIG. 81. Incoming light rays 766 impinge at normalincidence and proceed through the beam director 762 until reaching thewedge/air boundary. At this location the light rays 766 refract awayfrom the normal to the boundary per Snell's Law. The beam director 752and 762 can also take the form of a series of identical microprisms, asshown in FIG. 82 and described for the method of FIG. 27 (the elements402 and the deflection angle β). FIG. 81 is drawn in an explodedperspective to show, as one example, the film attachment of thepolarization selective reflecting film 754 and 758 to the prism 742 andthe films 756 and 760 to the prism 744. In addition, a splitterembodiment that can be used in locations where input light isconverging, includes a negative lens section 768, as shown in FIG. 83.Notice that the embodiments of FIGS. 82 and 83 are substantially similarto the basic embodiment of FIG. 81 except for the condition of inputlight which is converging in FIG. 83 and collimated in FIGS. 79 and 83,and the form of the beam director element, which is prismatic in FIG. 82and wedged in FIGS. 79 and 3. Each embodiment includes crossed selectivereflecting layers 754, 758, 756, and 760 (see FIG. 81), which preferablycomprise the layers 754 and 760 aligned to transmit light ofpolarization P1 and reflect light of polarization P2. The layers 758 and756 are aligned orthogonally, so as to transmit light of polarization P2and reflect light of polarization P1. As shown, the layer 754 isseparately applied to the upper hypotenuse surface of the prism 742, andthe layer 758 is attached to the lower hypotenuse surface of the prism744. Conversely, the layer 756 is separately applied to the upperhypotenuse surface of the prism 744 and the layer 760 to the lowerhypotenuse surface of prism 744. These selective reflecting layers 754,756, 758 and 760 can also be any conventional dielectric multi-layercoating having the above described polarization splitting properties,although the use of wide band material is preferred in applicationswhere post projection lens beam angles in the refractive medium of thebeam splitter 22 can be as large as ±15 degrees.

Illustrative light ray 770 within the input beam 726, as shown forexample in FIG. 81, enters the beam splitter 22 heading left-to-rightalong the optic axis 100. When the ray 770 first strikes the properlydesigned selectively reflecting layer 754, approximately one half itsintensity is reflected downwards as ray 772 in polarization state P2 andhalf is transmitted to the right as ray 774 in polarization state P1. Onits downward path, substantially all of the ray 772 passes out as partof the lower polarized beam. The ray 774 in polarization state P1 isreflected upwards by its interaction with the layer 756 as the ray 766,and continues upward as part of the upper polarized beam 778. Any traceamount of polarization state P2 in ray 774 is transmitted by the element756 as ray 780, which also contains any P1 that fails to be reflected.This ray flux is removed from the optical system 10 and cannotcontaminate the output imate quality. When such an element is used atthe output of the projection lens 20, as envisioned for example, inFIGS. 1A-C, the prism element size D′ is given by:$D^{\prime} = \frac{a}{{1 - {2\quad \tan \quad \varphi_{m}}}\quad}$

where a is the diameter of exit pupil (see, for example 782 in FIG. 80)of the projection lens 20,φ_(m) is the extreme ray angle in the plane ofview (see for example 783 in FIG. 80) in the refractive medium. Hence,for previous examples of the projection lens 20 with ±35 degree maximumangle in air, and the exit pupil 782 of 0.2″, the minimum beam splittersize, D′, is about 1.25″ on a side.

It is also preferable, though not required, to practice all the opticalsystem inventions described with highest possible projected imagebrightness. To do so, there are three primary factors influencingoverall projection efficiency and brightness, that should be optimized,whether individually or together: (1) the cross-sectional shape of thebeam illuminating the SLM aperture, (2) the polarization of theilluminating beam, and (3) the efficiency with which light emitted bythe light source 12 can be utilized by the projection screen 26constrained by the SLM 14 and projection optics. Despite the wide rangeof advancements available, today's rear projection system productsremain extremely inefficient, with lamp to screen efficiencies typicallyno higher than 5-10%.

Beam shape is a particularly important factor in achieving good screenefficiencies. One reason for this is that matching the illuminating beamshape to that of the rectangular SLM aperture offers a potential gain inscreen brightness over ordinary projection systems of 1.64. Anotherreason is that conventional beam-splitting methods for achievingpolarized illumination suffer serious uniformity deficiencies when usingcircular as opposed to rectangular input light beams. Without the meansto improve beam-shape, the beam-splitting methods of polarizationcontrol are largely impracticalThe availability of efficiently-polarizedlight is important preferred embodiments of the polarization-dependentprojection system 10 inventions introduced above. Efficient polarizationcontrol is also advantageous, in general, as it offers a gain in screenbrightness for polarization-dependent LCD-type SLMs of as much as 2.0over conventional unpolarized systems.

Accordingly, the corresponding potential for overall efficiencyimprovement in a projection system is significant. Combining theaforementioned performance gains from beam-shaping and polarizationrecovery, without loss, implies a potential improvement in screenbrightness over conventional systems approaching a factor of about 3.Then, incorporating additional means for improving the percentage oflight flux that can be passed from the light source 12, through theshaping means, through the polarization recovery means, through thefolded-optic projection system and to the projection screen 26, affordsthe potential for even greater performance gain in comparision with thatof conventional methods.

Each of the three components of a projection system's screen brightnessare hereafter described in sequence: Beam-Shape, Polarization Recovery,and Flux-Utilization.

The potential efficiency improvement possible from beam-shaping alone,can be understood from the following discussion. Projection systemsusing the standard TV 4:3 aspect ratio with circular illuminationsources, waste 39% of the incident light, as this much energy fallsoutside the inscribed 4:3 rectangle. If this wasted light could berecovered and recycled usefully within the inscribed 4:3 rectangle,doing so would increase the rectangle's flux density by 64%. Suppose a100 W arc lamp (such as the Philips.MHD 200 c) generating 6000 lumens isused in a conventional LCD based projection system design, and that as aresult 250 lumens of light flux falls usefully on the projections screen26 (4% efficiency). In this case, if the system's circular output beamcontained 1000 lumens before entering the LCD aperture 14, as forexample in FIGS. 61 and 63, 500 lumens would be discarded by the LCD'spolarizer, and of the remaining 500 lumens, only 61% or 305 lumens wouldbe passing through the rectangular aperture and would be available forthe projection screen 26. With only a 60% efficient approach fortransforming and recycling (rather than truncating) the system'scircular beam cross-section, 60% of the formerly truncated lumens, or117 lumens, could be added to the 305 lumens or available flux, leadingto a potential brightness gain of 1.4. (A 70% efficient approach wouldlead to a brightness gain of 1.45.) Given the implied projectionefficiency of 82%, the 100 W lamp would generate 346 lumens rather thanthe 20 lumens without this beam-shape transformation. Then, with an 80%efficient means to recover the 500 lumens of wasted polarization, andthe same ratios as before, 328 additional lumens can be transmitted tothe projection screen 26, raising the total screen lumens to 674 lumens,a combined improvement over the original 250 lumens of 2.7. If 250lumens were considered an adequate number for the optical system 10, thesame result can be obtained, not with a 100 W arc source, but ratherwith a comparably efficient (60 lumens/watt) 37 W arc light source 12.Accordingly, using a 50 W arc source, one would expect to yield 337lumens on the projection screen 26, which is still 35% more screenbrightness than is generated with the unimproved conventional systems100 W source. Lower wattage arc sources are generally preferred forseveral reasons. Aside from the implied energy savings, lower wattagesources have longer operating lifetimes and contribute less heat.

An efficient method for converting a light beam of circularcross-section to rectangular cross-section is described in FIGS. 90-91,using reciprocating mirrors 824 (824B) and 830 (830B) that re-cycleotherwise wasted light from the periphery of the circular output beamand into the central core of the correspondingly rectangularly-shapedoutput beam. These reciprocating mirrors 824 and 830 operate inconjunction with the conventional paraboloidal or ellipsoidalilluminators illustrated in FIGS. 88 and 92, using the conventionalglass-enclosed arc discharge light source illustrated in FIG. 89, andthey do so without passing any of the recycled light through or near thearc. Perspective views of a conventional arc source's physical structureand near-field radiant distribution are shown in FIGS. 89A and 89Brespectively. Conventional beam-shaping methods are described by FIGS.86 and 87.

The embodiments of FIGS. 90 and 91 avoid problems of returning raysthrough the arc region 833 (see FIG. 86), and also use a reciprocatingmirror design arranged so as to both recycle light and preserve beamuniformity. The example embodiment of FIG. 91 uses a negative lens 812to pre-collimate output rays 814 for beam displacement, and a positivelens element 816 to re-converge the displaced rays to an appropriatefocal point 818.

Using the embodiment of FIG. 90A as an example, light from the standardlight source 12, which can be the ellipsoidal illuminator system 808 ofFIG. 92, or the aspherized ellipsoidal systems described hereinafter, iscollected from the output of FIG. 92 and directed towards the lens pupil817 at the nominal focus 822 of the ellipsoid. A circular mirror ofhyperboloidal or modified hyperboloidal form 824, with an axial apertureof rectangular cross-section matching the shape of the SLM 14, reflectslight to the smaller concave (or convex) mirror 830 (or 830′ in theembodiment of FIG. 90D). At this point the light is reflected by thesmall mirror 830 so that it is also directed towards the nominal focus822 of the ellipsoid and the entrance pupil 817 of the system'sprojection lens 20. This arrangement is made feasible by theincorporation of a beam expander 844, which will be described shortly,as in for example FIGS. 97 and 98. The beam-expander 844 takes theinterior (or formerly occluded area) in the center of the light beamproduced by the light source 12, which can be the ellipsoidal arc sourcesystem 808 of FIG. 92 and then expands it to accommodate the light addedby the small mirror 830 (or 830′), so that the overall etendue ispreserved and so that there are no localized peaks in power density. Bythis manner, maximum use is made of the available light, as light thatwould have otherwise been unable to pass through the aperture of the SLM14 is re-routed, as for example by the mirror set 824 and 830, throughthe SLM 14 and towards the entrance to projection lens 20 in a usefuldistribution.

In the first stages of designing such a light recovery system, themirror 824 begins with a hyperboloidal form, but is then refined furtherto take on a modified form that preserves beam uniformity. The smallmirror can be concave (830) or convex (830′) and have a hyperboloidal ora modified hyperboloidal contour. These mirrors can also have anellipsoidal or modified ellipsoidal contour, can be segmented, facetedor Fesnelized.

Arranged in the simple illustrative manner of FIG. 90A, a peripheral ray840 is re-directed by the mirror 824 as ray 842 passing through thepoint 828 (or 828′), and is then re-directed by the mirror 830 (or 830′)towards the focal point 822. As such, the peripheral ray 840 istransformed to an interior ray fitting within an occluded spatial zone832.

The output light distribution from the mirror 830 mimics that of thelight pattern on the reciprocating mirror 824, where incident light suchas the ray 840 strikes one of the four peripheral crescent sections824A, 824B, 824C or 824D (see FIG. 90C). Unless deliberately altered,the output distribution from the mirror 830 then has a rectangularinterior dark zone corresponding to and proportional to the rectangularclear aperture 826 of the mirror 824. More significantly, the power (orflux) density that results in the four reduced-size crescent sections827A, 827B, 827C and 827D in FIG. 90B located within the field of mirror830 (or 830′), becomes significantly higher than the correspondingdensity within the surrounding beam areas. The overall beam profile isshown schematically in FIG. 90B for a cross-section along line B—B justto the right of the beam expander 844 in FIG. 90A. While there can besome applications that can withstand such a locally-skewed interiorlight distribution, it is generally preferable in most applications ofthe projection systems 10, to arrange for the flux in these crescentareas to be re-distributed evenly (or substantially evenly) throughoutan interior light circle 835 shown in FIG. 90B, otherwise filling in theintrinsically vacant rectangular hole.

There are two basic steps to redistributing this light uniformly withinthe field of mirror 830 (or 830′). The first step, anticipated above, isthat the entire beam is expanded by means of the beam-expander 844 sothat the average flux density within the expanded interior light circle835 approximately equals the average flux density in the exteriorportion of the beam. The second step involves corresponding mirror shapechanges that cause the light distribution of the reduced size crescentimages 827A, 827B, 827C and 827D (see FIG. 90B) to be re-arranged withinand throughout the region of light 831 projected by mirror 830 (or830′). This re-arrangement can be accomplished by one of severalpossible means, each acting to distort or re-structure the crescentimages on the mirror 830 (or 830′) so that they take up more of theavailable interior region 831 of the light circle 835. One means fordoing so involves modifying the functional shape of one or both of theconicoidal mirrors 824 and 830 (or 830′) by means of their aspherizingterms to cause the crescent patterns to become purposefully distortedand overlapping. Another means involves segmenting, faceting orfresnelizing the surfaces of one or both the mirrors 824 and 830 (or830′) so that there is a deliberately designed distribution of the focalpoints 828 (or 828′), and so that the resulting light distribution onthe surface of the mirror 830 (and within the light circle 835) is not asharply focused image. A third and most preferable approach is toarrange to systematically blur the focusing precision of thereciprocating mirrors 824 and 830 (or 830′) so that the points for thesharply-focused crescent images are not only blurred, but selectivelyblurred. This latter de-focusing method will be described in greaterdetail, as follows.

The reciprocating mirror method described above is applied to closelymatch the shape of the beam of light rays to the rectangular shape ofthe SLM 14 when the rays cross the plane of the SLM 14. It is preferablethat any inhomogeneities developed within the rectangular cross-sectionbe eliminated or minimized. The surpression of non-uniformity isachieved by means of secondary mechanisms that are applied to create thelocalized non-imaging behavior that blurs or evens-out any region ofnon-uniform flux densities, such as those of the crescent areasdiscussed above.

Each point on the SLM 14 is illuminated by a finite cone of light rayssuch as that meeting the requirements of an f/2.5 form of the projectionlens 20. As a result, the aperture structure of the ellipsoidal (ormodified ellipsoidal) illuminator 808 of FIG. 92 is, for example,pre-determined by surrounding every marginal point on the rearwardprojection of the principal rays through the margin of the SLM 14 with asmall circle whose diameter is set by the f/number of the projectionlens 20. The illumination system's circular output aperture is madelarge enough to include the combined area generated by the sum of thesesmall circular areas of light.

The four outlying crescent areas 829A, 829B, 829C and 829D in FIG. 90Bare defined by the area difference between the rotationally symmetricillumination system's circular output aperture, as above, and the innerarea corresponding to the rectangular shape of the SLM 14. The combinedcrescent area can be seen to represent 39% of the overall circular beamarea for the 4:3 rectangular aspect ratio used in the above examples.

In order to make use of the substantial amount of light contained inthese crescent-shaped areas, the size of the circular region into whichthis flux is to be deposited is expanded, as taught above, so that theresulting expanded area equals that of the four crescent areas referredto above, namely the sections 824A, 824B, 824C and 824D. With thismodification, the most efficient transfer of light energy from theseout-lying crescent areas to the expanded interior region occurs when theentendue is preserved, a condition satisfied when a substantiallyuniform distribution of light is pre-arranged within the expanded area.

Beam uniformity is achieved by making corresponding shape modificationsto one or both the reciprocating mirrors 824 and 830 (or 830′).Specifically, the curvatures of the segments of the mirror 824 arechosen so the contour generated by the principal rays encountering thesesegments, is a reduced and deliberately “blurred” image of the lightpattern falling on the larger mirror segments. If only principal raysare taken into account, the result would be a sharply-focusedilluminated area on the small mirror 830 (or 830′) which has arectangular clear area of the same proportion as that of the mirror 824.Since additional rays surround each principal ray due to the finiteaperture of the projection lens 20, the imagery on the small mirror 830is not point-to-point, but rather point-to-circular area. Because ofthis, the resulting imagery is intrinsically “blurred,” and therectangular clear area can be made to have a more uniform distributionof light because of the calculated overlaps of these areas of light.Preferably, the degree of intrinsic “blurring” is deliberately increasedand directed so as to achieve a substantially uniform lightdistribution. The forms of the mirror crescent sections 824A, 824B,824C, and 824D are individually adjusted such that a highly distortedlight mapping is carried out by the principal rays. Then the combinationof this adjustment with the aforementioned point-to-area mapping causedby the surrounding rays is used to secure the preferred degree of evenillumination in the pupil of the projection lens 20 for all points inthe area of the SLM 14.

A corresponding adjustment of the small mirror 830 (or 830′) contour isalso made to ensure that together with an even filling of the smallmirror area that there will be a properly controlled angulardistribution of radiant energy.

In yet a further embodiment of this general light shaping method, thebeam-expander 844 can be used that creates a vacant area strip (orstripe), rather than the vacant area circle of FIGS. 90 and 91, andcorrespondingly, the reciprocating mirror 824 with the rectangular clearaperture 826 is replaced by one or two pairs of flanking cylindricalmirrors.

Another arrangement is shown in FIG. 93A using the light source 12 asthe paraboloidal illuminator system 897 of FIG. 88. In this embodiment,the outer reciprocating mirror 824P has a paraboloidal or modifiedparaboloidal surface with the focal point 828P (or 828P′), and thesmaller interior mirror 830P (or 830P′ in FIG. 93B) also has aparaboloidal or modified paraboloidal surface with the common focalpoint 828P (or 828P′).

An additional embodiment is described in FIG. 94 for paraboloidalilluminator systems 810. (The same approach can be applied to theellipsoidal illuminator 808 of FIG. 92 by inserting a negative lens toweaken or eliminate the ellipsoidal convergence.) The embodiment of FIG.94 uses the paraboloidal or modified paraboloidal reflector 848 tocollect a significant angular fraction of the flux re-directing thiswide angular range into a collimated output beam of circularcross-section that is output through the rectangular aperture 826 in thelarger reciprocating mirror 824E. Preferably, the circular cross-sectionextends beyond the rectangular aperture 826 so that the resulting outputbeam is rectangular in cross-section. Doing so, causes that portion oflight striking the reciprocating mirror 824E to be re-directed backtowards the smaller reciprocating mirror 830E. Light rays missed by theparaboloidal or modified paraboloidal reflector 848 are also re-directedby the larger reciprocating mirror 824E to smaller reciprocating mirror830E. So that the smaller reciprocating mirror 830E can re-direct bothsources of re-cycled light, as above, to the interior portion of theoutput beam, the form of the larger reciprocating mirror 824E is made insections, as will be described hereinafter. The beam-displacer 844 isprovided to apply the correct amount of beam diameter expansion so thatthe power density of re-directed light matches the power density oflight collimated by paraboloidal collector 848. A front view ofembodiment of FIG. 94A as seen from the plane perpendicular to the lineC—C in FIG. 94A is shown in FIG. 94E. The view in FIG. 94E shows themajor sections 824E1-5 of the larger reciprocating mirror 824E, theoutput aperture 848′ of the ellipsoidal or modified ellipsoidalreflector 848, and the output aperture 830E′ of the smallerreciprocating mirror 830E. The outer toric section 824E5 of theellipsoidal or modified ellipsoloidal mirror 824E, receives light raysdirectly from the arc source 833 and its focal point 850, and re-directsthose light rays towards the first focal point 852 of the correspondingportion of the smaller reciprocating mirror 830E. The smallerreciprocating mirror 830E is paraboloidal or modified paraboloidal, witha second focal point at infinity. Accordingly, in this example, there-directed output rays from the smaller reciprocating mirror 830E aremade to run parallel to those of the paraboloidal or modifiedparaboloidal reflector 848. The inner crescent sections 824E1, 824E2,824E3 and 824E4 of larger reciprocating mirror 824E, receives light raysthat have been re-directed by the paraboloidal or modified paraboloidalreflector 848 that are substantially collimated. Accordingly, thesemirror sections have a different shape than the mirror's outer toricsection 824E5. In this case, the inner crescent sections 824E1, 824E2,824E3 and 824E4 are designed to re-direct the in-coming collimated lightrays towards focal point 828E, whereupon these rays will be ouput ascollimated rays as shown in the magnified cross-section of FIG. 94D.Also, see the detailed portions of this embodiment in FIGS. 94B and 94C.Additional modifications to the shape of one or both the reciprocatingmirrors 824E and 830E, including that of the individual sections asdescribed above, are made to maximize beam uniformity in the same manneras illustrated for the embodiment of FIGS. 90-93.

It can be even more preferable to expand the beam 854 first and thenperform the reciprocating mirror beam shape transformation, as done inFIGS. 95 and 96 shown for the paraboloidal illuminator system 808 ofFIG. 88. This arrangement of elements leads to an integratable package,and is taken with the ellipsoidal illuminator system 808 of FIG. 92 aswell, using the negative lens 812 as a pre-collimator. In FIG. 95,exterior mirror 856 is a paraboloid with focus at 858; and interiormirror 860 is, for example, a paraboloidal sector with focus at thepoint 850, although other forms are equally possible. FIG. 96 representsthe case where the interior mirror 862 is convex. This format isadvantageous as the virtual focal point can be located within the beamdisplacer 844 without interference. In either case, it is possible, asin FIG. 96, to design the system with two foci, 864 and 866.

In the numeric example provided hereinbefore, it was estimated thatthere might be 500 lumens in the circular output ray bundle 846 of FIG.90 and that 61% or 305 lumens would pass through the rectangularaperture 826. Another 23.4% would be available after reflection andother losses for recycling and redistribution within the occludedspatial zone 832. This implies that the occluded spatial zone 832 wouldneed to accommodate 117 lumens. Ordinary occluded zones are not expectedto be larger than about 3 mm in diameter at plane 868 in FIG. 90 whenthe diameter of the concave mirror 915 is proportionally about 20 mm.Accordingly, there would be a dis-proportionally higher flux density inthe occluded spatial zone 832 (about 1600 lumens/cm²) than in therectangular output aperture 826 (about 160 lumens/cm²), which isimpractical. With this flux density differential left uncorrected, thearrangements of FIGS. 90, 91, 93 and 94 would each exhibit a significant(10×) hot spot in the center of the output ray bundle 846 that wouldcarry forward through the optical systems 10 of, for example, FIGS. 1A,7-13, 20, 21, 32-38 and 54 and appear as a center brightness peak on theprojection screen 26.

A preferred way to adjust for this imbalance on the projection screen 26is to physically enlarge both the illuminator's output rectanglediameter and simultaneously the diameter of the occluded spatial zone832 (see FIG. 91). For the numerical example used above, enlarging theoccluded spatial zone 832 to 9.65 mm, and proportionally enlarging theoutermost beam diameter, balances the inner and outer flux densities,and yields a uniform output ray bundle profile (average flux density ofabout 160 lumens/cm²). Another approach would be to adjust the opticalpower of the concave mirror 830 (or 830′) (see FIGS. 90A and C) so thatthe redirected rays have a proportionally larger output angles, yetstill fall with the range where they would be able to pass through boththe SLM 14 aperture and the entrance aperture of the projection lens 20.Of these two approaches, which can be applied separately or incombination, it is typically more efficient to enlarge the beam diameterby means of the beam displacer or expander 844.

One way to expand the light beams 846 of the type in FIG. 90A is toapply the collimated light prismatic beam-displacement method of FIGS.26-28, which in one example, the Fresnel-like radially-grooved prismaticfilm element sheets 402 and 406 separated by the gap, g, were used forthe opposite purpose, to reduce a beam's diameter. While the systemdeveloped in FIGS. 26-27 functions in both directions, and a light beamincident on the prismatic film layer 406, for example, would exit theprismatic film layer 402 with a larger diameter, some inefficiency wouldbe caused by light rays falling undesirably on prism side facets such asside facet 872, rather than on the hypotenuse facets such as facet 870(see FIG. 97A). A more preferable arrangement for beam expansion by thismethod is shown in FIGS. 97A and 97B has each of the prism film layers402 and 406 in FIG. 27 rotated by 180 degrees about the horizontal axisforming a new set of prism film layers 876 and 878 respectively, so thatin this orientation the prism's side-facets 872 do not come into play

Consider light ray 874 in FIG. 97 incident on the prismatic layer 878 atnormal (or near normal) incidence. This light ray 874 passes through thesecond prismatic layer 878 and refracts into air at an angle to thenormal, γ, given by

 γ=β−α

where α is the prism angle (the same was assumed in this case for boththe prismatic layers 876 and 878), n is the prism refractive index and

β=arcsin(n sin α)

Accordingly, the relationship between beam expansion ρ and the gap, g′,becomes

ρ=g′tan γ

For a 30 degree form of the prismatic layer 878, the gap, g, associatedwith a 9.65 mm displacement (4.825 mm on each side) is 12.6 mm, which isan extremely compact solution. The combination of this system within theembodiment of FIG. 93 is illustrated in FIG. 97C.

The prismatic film layer 876 and the second prismatic film layer 878 canbe formed with either be macro-sized or micro-sized prisms (as in thediamond-cut grooves typical of Fresnel-type lens elements or theso-called Brightness Enhancing Film (BEF) as manufactured by 3MCorporation). The only limitation is that the prism periodicity shouldbe chosen to avoid optical interference from Moire patterns which can begenerated between the two prismatic film layers 876 and 878, as well asbetween these elements and the SLM 14. Common methods of Moire avoidanceinclude making each elements prism period different, and making theprism periods sufficiently smaller or larger than the SLM 14 pixeldimensions (10-20 microns).

One other example of a means for enlarging the occluded spatial zone 909(see for example FIG. 98B) is shown as refractive element 880 in FIG.98A. The example of collimated input rays 882 is used for simplicity,and the same reciprocating mirror method illustrated in FIG. 97C. Thecollimated input rays 882 can always be provided either by theparaboloid system 810 of FIG. 88, or by using a negative lens (notshown) at the output of the ellipsoidal system 808 of FIG. 92. Therefractive element 880 that enlarges the central zone can be formed ofany suitable transparent plastic or glass material. In one embodimentshown in FIG. 98B, the refractive element 880 is located preferablydirectly to the right of the larger reciprocating mirror 824. In thespecific example of a 20 mm beam diameter, and forming the element usinga medium of refractive index 1.5, the overall length, L, as in FIG. 98A,of the conic element along the optic axis 100 would be 22.59 mm (or0.89″). It is also possible to locate the refractive element 880 to theleft of the concave mirror 824, but the element's shape would preferablybe modified to account for the more complicated ray paths.

Another method for efficiently transforming the shape of the circularoutput ray bundle 854 or 846 (see FIGS. 88, 90 and 92 for example)produced by the ellipsoidal or paraboloidal light source reflectorsystems 808 and 897, is depicted in FIG. 99A for the ellipsoidal lightsystem case. The method of FIG. 99A consists of a converging output lens884, to provide for proper focal point F for the projection system 10.The circular bundle of the converging input light 846 fills the inputaperture 886 of a well-matched lightpipe 888 of circular inputcross-section that has been formed of glass or plastic. Thecross-sectional area of this lightpipe 888 is pre-formed to a shape thatextrudes mathematically from circular to rectangular, and preferablydoes so adiabatically, over a necessary length 890 so that there isminimum associated loss from either the scattering caused by too abruptslope changes or from any associated total internal reflection (TIR)failures caused within the lightpipe 888 during the process. Severalillustrative cross-sections are shown as 892, 894, 896, 898 and 900 inFIGS. 99A and 99B. Once the necessary shape transformation has beeneffected, or as part of the adiabatic shape transformation process, thelightpipe's diameter is increased in a prescribed way so that thecalculated cross-sectional profile of a non-imaging optical angletransformer 902 is developed with end face 904 (conventionally referredto as a Compound Parabolic Concentrator, “CPC”). It is the designedproperty of this angle transformer 902 that ray bundle 906 at itsentrance aperture cross-section 908 propagates in the dielectric element902 by TIR (total internal reflection) at the sloped sidewalls 1212formed by the dielectric air boundary layer, such that theangular-aperture area transformation equality known as the Sine Lawoperates or substantially operates between the aperture cross-section908 and the end face 904 as: A₁Sin²θ₁=A₂Sin²θ₂, where A₁ is therectangular cross-sectional area at the cross-section 908, θ₁ is thehalf angle of the ray bundle 906, A₂ is the rectangular cross-sectionalarea of the end face 904, and θ₂ is the half angle of output ray bundle910.

Both the shape of the CPC sidewall and the output angle of the raybundle 910 can be modified by optionally including the converging lenselement 884. Elements represented schematically by FIGS. 99A and 99Bhave been designed and analyzed using Breault Research Organization,Inc. optical modeling/tracing software ASAP, and were found to havepractically no geometrical conversion loss between the circular andrectangular cross-sections indicated.

Once the illumination source has been so arranged to have a rectangularbeam cross-section, the methods for doing so can be combined with one ofa number of split-beam polarization recovery and color sequencingmethodsto deliver a deliberately polarized beam of rectangularcross-section suitable for the optical systems 10 of FIG. 1A, 7-13, 20,21, 32-38 and 54.

In such modifications, it is further desirable to utilize efficientcollimated, unpolarized light sources making use of the beam-shapingmethods described above. Therefore, four collimated, unpolarizedrectangular light (CURL) source arrangements 916, 918, 920 and 922 aresummarized schematically in FIGS. 100-103, based on the variousembodiments described hereinbefore. Each contains either theparaboloidal or modified paraboloidal reflector 848 for the arrangements916 and 920 or the ellipsoidal or modified ellipsoidal reflector for thearrangements 918 and 922, the arc source 833, the reciprocating mirrorset, such as for example, 830 and 824, 830P and 824P, or 862 and 856,the beam expander 844, and in addition for the case of the arrangements918 and 922, the negative collimating lens 812. When combined with amethod for purely polarizing each system's unpolarized output, acollimated and purely polarized beam having rectangular cross-section isso generated. Such purely polarized light sources 12 are highlypreferred with the above polarization-dependent image projection system10 inventions, to obtain bright, uniform, and ghost-free projectedimages.

A conventional polarization recovery system is shown in FIG. 104 forgenerating a polarized output beam. A preferred wide band polarizationrecovery system suitable for use with the projection systems 10utilizing the CURL sources 916, 918, 920 and 922 is illustrated in FIGS.105 and 106. In the embodiment of FIG. 105, one of the four CURL Sources916, 918, 920 and 922 is combined with a polarizing beam splitterconsisting of preferably, a wide band 3M-type polarization selectivereflecting or beam splitting film 926, such as for example layers 116and 118 in FIG. 1A. Also included are layers 754, 756 758 and 760 inFIG. 81, and in FIG. 105 four Porro prisms 924, 928, 930 and 932, threeabsorption type polarizers 934, 936 and 938 as discussed above with theabsorption polarizer 934 blocking P2, the polarizer 936 blocking P1 andthe polarizer 938 blocking P1, the SLM 14 with the buffer zone 148, andin this case, telecentric projection lens 940. In the embodiment of FIG.106, a second beam-splitter 22 is used to re-direct the light at the SLM14 output orthogonal to the original direction and in oppositedirections, each to an upper and lower telecentric projection lens 946and 948.

In FIG. 107 is shown another embodiment for efficiently pre-polarizinglight generated by the converging-type light source 12, rather than thecollimated-type light source 12. An acceptable form of the convergingsource 12 can be the ellipsoidal system 808 of FIG. 92, the paraboloidalsystem 897 of FIG. 88 with a converging or condensing lens (such as, forexample, a plano-convex lens) or any one of the CURL-type sources 916,918, 910 and 922 of FIG. 105 with such a converging or condensing lens.This embodiment uses two reciprocating reflecting elements 956 and 962,the element 962 being arranged in conjunction with refractive media 974Aand 974B, the element 956 being arranged with a small light inlet hole954. Together, the elements 956 and 962 selectively pass, convert andrecycle polarized light so as to convert unpolarized input light rayssuch as ray 952 to polarized output light rays such as ray 964. Theillustrative input light ray 952 passes through focus at or near thesmall physical hole 954 in the first reflecting element 956 which iscentered on the rays point of convergence. The reflecting element 956 iscomposed of two layers: a metal or metallic reflective layer 958 (seeprevious description of the polarization handedness conversion at ametal or metal-like film layer)) and a preferrably quarter-wavepolarization retardation layer 960 (see previous description of the wideband retardation layers). The reflecting element 962 is composed of asingle wide band polarization selective material that passes P1 andreflects P2 (see previous descriptions of wide band polarizationselective reflecting materials). The illustrative ray 952 continuesleft-to-right through the hole 954 and the refractive media 974A untilit strikes the second reflecting element 962, whereupon it is split intotwo orthogonally polarized rays 964 and 966. The polarized ray 964 istransmitted left to right in polarization state P1, and the otherpolarized ray 966 is back-reflected towards the polarization-convertingand reflecting element 956 in the orthogonal polarization state P2. Theback-reflected polarized ray 966 on approaching the reflecting element956 first passes right-to-left through the polarization retardationlayer 960, strikes the polarization converting reflective layer 958,which converts polarization state (right hand circular to left handcircular and vice versa) and redirects the ray back towards the secondreflecting element 962 as P1 ray 968, orthogonal to the polarization ofthe ray 966. As such, the orthogonally polarized ray 968 passes throughthe reflecting element 962 as output ray 970, having the samepolarization state P1 as the originally polarized ray 964. In effect,this mechanism develops two beams, one original and one recycled, havingthe same polarization states. The illustrative light ray 952 can be saidto have been polarized by the polarization selective reflecting element962, and the orthogonally polarized ray 966 said to have been recycledand converted to the same polarization as the original output light ray964. The relative shapes of the two reflecting elements 956 and 962, aswell as their positions can be adjusted, along with the associatedinclusion of other means of optical power, such as first and secondrefractive materials 974A and 974B, so that the two resulting outputpolarized rays 964 and 970 overlap in such a way that their compositebehavior is as of a single beam of light. For example, the polarizationselective reflecting layer 962 can be either a flat plane or a wealdycurved as a conicoid, with or without aspherizing terms. The firstreflecting element 956 can be a conicoid with or without aspherizingterms. The addition of aspherizing terms can be used as a means toprovide final adjustment on achieving sufficient the preferable amountof spatial beam uniformity or the preferable angular distribution ofrays or both.

In a further embodiment in FIG. 108 a first reflector 976 is aparaboloidal (or modified paraboloidal) section with a radius ofcurvature of 30 mm and a second polarization selective reflector 978(such as the previously discussed 3M type polarization selectivereflecting film) separated from the first reflector's paraboloidalvertex 980 by 2.5 mm. The polarization selective reflector 978 isfurther combined with a composite refractive element 982 whose centralportion 984 operates like a plano-convex lens, and whose peripheralportions 986 operate like plano-concave (negative) lenses. Incomingun-polarized light beam 988 converges to the aforementioned paraboloidalvertex 980, and then diverges symmetrically about the system's opticaxis 100 left-to-right towards the polarization selective reflector 978.Ray 992, for example, on striking the second reflector 978, is partiallytransmitted as, linearly polarized light ray 994 of polarization stateP1 within the refractive lens portion 984 and transmitted as output ray996 of polarization state P1. When the ray 992 strikes the secondreflector 978, the non-transmitted fraction is reflected, for example,as linearly polarized light ray 998 of the orthogonal polarization stateto P1, P2. This back-reflected light ray 998 of polarization P2continues left-to-right until it passes through the firstpolarization-converting layer 1000, in this case preferably aquarter-wave retardation film, and becomes left-hand circularlypolarized. When this so-converted ray 998 is re-directed by metalizedreflecting layer 1002, the incoming left-hand circularly polarized lightray 998 is converted to an outgoing right-hand circularly polarized ray1004 as has been described several times previously, which upon suchre-direction, passes back again through the converting layer 1000 and ispolarized as P1. The P1 polarized ray 1004 proceeds towards the secondreflector 978 at an angle determined by the surface contour of the firstreflector 976. On striking the reflector 978, the ray 1004 inpolarization state P1 is transmitted as ray 1006 and refracted withinnegative lens portion 986 of the composite lens 982, emerging as outputray 1008 within the upper output beam 1012. By design, the output rays996 and 1008 both appear to have come from (or very near) the originalpoint of entry at point 980. The result, when all the rays of theincoming unpolarized beam 988 are traced, is a single diverging outputbeam 1010 of a single (linear) polarization. A characteristic of thisoutput beam 1010 is that the peripheral rays 1012 are made up of rayswhose polarization is ordinarily discarded, but that, by virtue of thisdesign, have been recycled, converted and recovered as rays of usefulpolarization.

The embodiment of FIGS. 107 and 108 can be applied just as easily toproduce an output beam with the upper image region 82 polarized as P1and the lower image region 84 polarized as P2, which is the formpreferred for practice with the split-image optical systems 10. In thispreferred variation, instead of making the second reflector 978 acontinuous sheet of 3M-type material that passes P1 and reflects P2 asin both FIGS. 107 and 108, the element 978 can be made with twoorthogonally oriented portions, an upper portion of the element 978 thatpasses P1 and reflects P2, and a lower portion of the element 978 thatpasses P2 and reflects P1. The corresponding structure of the firstreflecting element 976 remains unchanged, however, since the element 976acts to re-direct incident light in its orthogonal linear polarizationstate, whether the incident state is P1 or P2. Equivalent combinationsof shaped converting reflectors like the first reflector 976 and lenscombinations separated by a flat or weakly-curved polarization selectivereflecting planes are equally feasible; for example, the arrangementsillustrated in FIGS. 50-55 can also be adapted for this purpose.

It is also most preferable for these embodiments that an output lenselement such as 1058 in FIG. 108 be used either to pre-collimate thediverging output rays or alternatively to bring them to convergence at apre-determined point. For example, consider the arrangement of FIG. 109,which combines the polarization recovery methods of FIG. 108 with thesimple unpolarized ellipsoidal light source 808 of FIG. 92 and thecollimating output lens 1058 as before. In this case, a collimatedoutput beam 1016 of circular cross-section is produced with either asingle or split polarization, depending on the form of the reflectingelement 978. In addition, in FIG. 109, cylindrical mounting sleeves 1018and 1020 are used to illustrate a particularly compact means forachieving the preferred co-axial and axial alignments of elements. Thislens-barrel mounting method facilitates the addition of further elementsand openings, as needed, for the general purposes of heat extraction,filtering and cooling.

A further variation on the embodiment of FIG. 109 is illustrated in FIG.110. This embodiment achieves both the rectangular beam-shapetransformation of FIG. 102, for example, and the polarization processingof FIG. 108.

Another embodiment is illustrated in FIGS. 111A and 111B, where thepolarizing arrangement of FIG. 108 is combined with the beam-shapetransforming method of FIG. 102. FIG. 111B also shows a perspective viewof the outer package that applies qualitatively to FIGS. 109 and 110 aswell, although neither of which has the output mirror 856 arrangementshown in FIGS. 111A and 111B.

Yet another embodiment is illustrated in FIG. 112 where the embodimentof FIG. 108 is combined with a variation on the general reciprocatingmirror beam shape transformation method of FIGS. 93 and 98B, but in thiscase with the reciprocating mirrors elements 838 and 824 located outsidethe ellipsoidal (or modified ellipsoidal) light source 808 of FIG. 92 asin FIG. 102, and using the beam expander method of FIGS. 98A and 98B.Converging light 1056 in FIG. 112 enters the polarization embodiment ofFIG. 108 as before and is collimated by the plano-convex lens element1058. The interior mirror 838 is mounted axially on (or just within) thelens 1056 surface, and is hidden within the shadowed or occluded region1015 of the interior output beam 1016 of the polarizing embodiment'soutput lens 1058. The collimated output bundle 1016 passes through therefractive beam expander 1062, which enlarges the beam diameter, and inparticular the diameter of the vacant beam interior as discussedpreviously, from in this case 1015 to 1017, as shown in FIG. 112. Rayson the beam 1016 periphery falling between the circular outer diameterand the inscribed 4:3 (or other) rectangular aspect ratio, are clippedoff by the mirror 824 and recycled to the mirror 838 as describedpreviously, and out the interior channel through the beam expander 1062.

In another embodiment shown in FIG. 113A, a first reflector 1022 is aconvex conicoidal reflecting surface parallel to a plane orthogonal tooptic axis 100 and located at the system origin, in this case ahyperboloid with focal points 1026 and 1028 at minus 5 mm and 15 mm. Asecond reflector 1030 is a selectively-reflecting plane (or weaklycurved) surface composed of the wide-band polarization selectivereflecting (or splitting) film discussed hereinbefore and separated fromthe first reflector's origin by a 5 mm layer or air-gap. In thisarrangement, reflector 1030 is composed of the polarization selectivereflecting layer 978 such as that used previously in FIG. 108, and it isapplied to a transparent substrate 979 made of glass or plastic forrigidity and support. The first reflector 1022 in one form of theembodiment has two polarization-converting layers, a metallicpolarization-converting film 1032 that changes the handedness ofcircularly polarized light as described earlier, and preferably aquarter-wave retardation layer 1034, such as the wide band retardationfilms described numerous times above.

A second form of the embodiment for the reflector 1022 is shown in FIG.113B. The polarization-converting, quarter wave layer 1034, rather thanconforming to the shape of the reflector element 1022, is placed just infront of the element 1022 as a separate plane. One advantage of thisform of the element 1022 is there is minimal chance of any conversioninefficiency caused by the orientation mis-matches in making a flatsheet conform and adhere to an even slightly curved surface. In thiscase, incoming and converging unpolarized beam 1036 is heading towardsthe first reflector's focal point 1028, but has been preprocessed, forexample by means of the beam expansion methods of FIGS. 97A and 98A, toenlarge the beam's interior angular acceptance hole 1040 sufficiently toaccommodate the size of the first reflector element 1022, which isotherwise opaque. Illustrative principal ray 1042 converges towards thefocal point 1028, passing left-to-right above the reflector 1022 andheading towards the second reflector 1030. When the principal ray 1042reaches the second reflector 1030, it splits into two orthogonallinearly polarized rays, a reflected ray 1044 of polarization P2, and atransmitted ray 1046 of polarization P1. The reflected ray 1044 isredirected back towards the first reflector's other focal point 1026,but strikes the first reflector 1022 on the way. When the reflected ray1044 reaches the first reflector element 1022, it passes through thequarter wave polarization-converting layer 1034 and becomes, in thisexample, left-hand circularly polarized. Upon striking the metallicpolarization (handedness) converting layer 1032, the reflected ray 1044then becomes right-hand circularly polarized and is redirected back tothe right, passing once again through the quarter wavepolarization-converting layer 1034, and emerging as output ray 1047 withthe orthogonal linear polarization state P1, which on reaching thesecond reflector 1030, is transmitted within the previously unoccupiedinterior region 1040 as ray 1049. Accordingly, all such raysselectively-reflected as P2 at the second reflector 1030 aresubsequently converted and redirected by the first reflector 1022, so asto be recycled within the interior core of output beam 1048.

As mentioned above, the input beam's interior core 1040 is preferablyexpanded to make room for these recycled rays and to make sure that therecycling reflector element 1022 is hidden within the expanded shadowregion, by either the method of FIGS. 97A or 98A. Since the beamexpanders 880 of FIGS. 97 and 98 are preferably used with collimatedlight, and since the method of FIGS. 113A and 113B requires converginglight, an alternative arrangement such as that in FIGS. 114 or 115 usingcollimated input light is generally preferred. For example, in theembodiment of FIG. 114, the natural interior occluded spatial zone 832of the paraboloidal light source 810 is pre-enlarged by the action ofthe refractive beam expander element 880 to a diameter 1052, sufficientto shadow the polarization converting and re-directing first reflector1022, which is mounted axially on converging (or condensing) lens 1054.The surface shape of the reflector 1022 is made such that its virtual(back) focus is at the focal point 1026 and its front focus coincideswith the lens element 1054's point of convergence, the focal point 1028.Additional compactness is then achieved by truncating the apex of theexpander element 1050 nearly to the edge of the expanded shadow diameter1052, and by mounting the re-directing reflector element 1022 directlyon the converging lens 1054. The same barrel-mounting methods of FIGS.109-112 are applied just as advantageously for these embodiments. The3M-type polarization selective reflector (or beam splitter) used in, thereflector 1030 of the example embodiments of FIGS. 114 and 115, consistsof two film sections, an upper layer 1064 that passes P1 and reflectsP2, and a lower layer 1066 that passes P2 and reflects P1. Because ofthis orthogonal film orientation structure, the output polarizationdistribution is half P1, half P2, and thereby is appropriate for thesplit-image projection system 10 methods described above. If thereflector element 1030 were covered with either the upper or lowerlayer, 1064 or 1066, over its entire support substrate 1078, the outputdistribution would have a single polarization, and would therefore notbe suited for use with the split-image projections systems 10 above. Inaddition, polarization filter clean-up layers are applied as upper cleanup layer 1068 and lower cleanup layer 1070. For the case illustrated,the upper clean up layer 1068, is made to block P2, and the lowercleanup layer 1070 is made to block P1, assuring polarization purity foruse with the split-image projection systems 10. A similar approach canbe taken to assure single polarization purity when using the embodimentsdiscussed hereinbefore with the single polarization projection systems10, such as for example the embodiments of FIGS. 14-25, 32-38, and50-55.

Since the current polarization processing methods are used with a meansof beam expansion to assure that incoming light is able to bypass theobstruction represented by reflector 1030, two preferable combinationsthat incorporate the rectangular beam-shape transformation methods ofFIGS. 97-98 are illustrated in FIGS. 116 and 117. In FIG. 116, thebeam-shape transformation method of reciprocating mirrors is employedwithin the paraboloidal system 897 of FIG. 88, as previously illustratedin FIG. 93A. Sufficient beam expansion is provided for by the refractivebeam expander element 880 of FIG. 98A so that substantially all there-cycled flux clears the polarization processing reflector element1030. The same approach is illustrated in FIG. 117, except that theprismatic film beam expander element set 876 and 878 of the methodillustrated in FIGS. 97A-C is used. The gain in efficiency that ispossible by such sequential recycling is illustrated by the principalray path 1072-1074-1076-1078-1080-1082-1082-1084-1086-1088 in FIG. 116.The arc light source 833 at the paraboloidal or modified paraboloidalreflector 848 focal point 850 outputs the principal ray 1072, which iscollimated or substantially collimated by the action of the paraboloid848. As this particular ray 1072 falls outside the rectangular beamshape desired, it is blocked by reflector 824 and re-directed throughthe focal point 828 as the ray 1076, to reflecting element 1092, whichthen re-directs the ray 1076 left-to-right parallel to the optic axis100 as the ray 1078. This ray 1078 encounters the conic beam expanderelement 880 and is refracted through it as ray 1080. When the ray 1080exits the element 880 into air, it becomes collimated as ray 1082 andrefracted by the lens 1054 as ray 1084, whereupon on reaching reflector1030 it is split into the two orthogonally polarized rays, with outputray 1094 of polarization P1 transmitted and reflected ray 1096 ofpolarization P2 recycled to the reflector element 1022, converted asbefore, re-directed as ray 1088 of polarization P1 and then transmittedthrough the element 1030 as recycled output ray 1098.

Another form of the polarization recycling of FIG. 113A is based oncollimated and converging input light embodiments illustrated in FIGS.118 and 119 respectively. In both cases, the conicoidal form of asmaller first reflector 1022′ made of the same construction as reflector1022 is hidden within the interior core 1040 of the input beam 1036 asbefore. In addition, a second reflector 1100 is a shaped conicoidalsurface, rather than as the plane or weakly-curved reflector 1030 ofFIGS. 113-117. This second reflector 1100 is arranged, in one case, withan interior reflecting layer 1099 of the 3M-type polarization selectivereflecting film that passes polarization P1 and reflects polarizationP2, and a transparent exterior support layer 1097. In the example ofFIG. 118 the pre-expanded collimated or substantially collimatedincoming rays 1036 bypass the first reflector 1022′ and first strike theconicoidal reflecting interior layer 1099 of the second reflector 1100.The layer is shaped as a paraboloid or modified paraboloid having afocal point 1101. The second reflector 1100 splits the directly incomingcollimated light rays 1036, outputting two sets of rays, one set ofcollimated rays 1037 of polarization P1 unchanged in direction, and oneset of reflecting or redirected rays 1039 of polarization P2 convergingtowards the interior focal point 1101. Before reaching this interiorfocal point 1101, however, this set of converging rays 1039 strike thesurface of first reflector 1022′, which is shaped in this case as anhyperboloid or modified hyperboloid with focal points at 1101 andinfinity. Having the same polarization converting structure andproperties as the reflector 1022, the reflector 1022′ receives rays ofpolarization P2 and outputs rays of polarization P1 heading back towardsthe first reflector 1100 in a collimated or nearly collimated beam, thatthen pass through the second reflector 1100 as collimated output rays1041 of polarization P1. The result is a consolidated output beam 1103of contiguous polarization P1, an annulus 1043 of the rays 1037 whosepolarization remained unchanged, and an interior region 1040 filled withthe rays 1041 whose polarization has been converted from P2 to P1.Arranging for the output beam 1103 whose upper and lower halves areorthogonally polarized is accomplished just as in the method of FIGS.114-117, by splitting the polarization selective reflecting layer 1099into a corresponding upper and lower half, an upper portion that passesP1 and reflects P2, and a lower portion that passes P2 and reflects P1,as has been described previously. The outer annulus of this beam 1103corresponds to those rays within the beam 1036. The beam expansion ispre-arranged so that the flux density within the interior region 1040equals the flux density in the annulus region of the beam 1036.

The example of FIG. 119 behaves analagously to FIG. 118, except that theincoming rays 1036′ are pre-arranged to converge towards a focal point1028′, and first and second reflector elements 1022′ and 1100′ areshaped as hyperboloids or modified hyperboloids respectively, with acommon focal point at 1101′ and 1028′. The incoming rays 1036′ are splitby the second reflector 1100′ into two sets of rays, one set 1037′retaining polarization P1 that continues converging towards focal point1028′ and another set 1039′ of polarization P2 that converges on thefocal point 1101′. The rays that are made to converge to 1101′, areconverted from P2 to P1, as before, and redirected towards 1028′ as rays1041′, filling the output beam's interior region 1040′.

The second reflector, whether 1100 or 1100′, contains an interior layermade from a wide band polarization selective reflectoring material, suchas the 3M dielectric multi-layer stack film discussed above. Similarly,the first reflector, whether 1022 or 1022′, contains an outer layer madeof a wide band (preferably quarter wave) birefringent-type phaseretardation film. Both these materials have preferred alignmentdirections. Because of this, their attachment to the curved surfaces ofthe reflectors 1100, 1100′, 1022 and 1022′, should be done thoughtfully.Rather than simply applying film sheets to smooth and continuousconicoids, the preferred embodiments will instead use faceted conicoidreflector element 1107, as shown in FIG. 120A, applying the associatedpolarization selective reflecting material 1108 as shown in FIG. 120B,pre-cut as elements 1104A, B, C, etc. to fit each facet 1102 in theideal orientation for the facet 1102. The ideal orientation 1109 isshown, for example, by the parallel arrows drawn on both the reflectorelement 1107 and on the material 1108. The more facets 1102, the moreefficient the associated performance and the more correspondinglydemanding the attachment process. Whenever the conicoidal reflectorelement 1107 is weakly curved, however, as in the example of FIGS.114-117, the inefficiency caused by directly laminating or deforming aplane sheet of film stock 1108 to fit the weakly-curved surface will beminor. Whenever the conicoidal surface of the reflector element 1022 isdeeply curved, as in the example of FIGS. 118 and 119, the facetedapproach is preferable. Although film attachment to such facetedsurfaces is considerably more challenging than film attachment to planesurfaces, an automated process for doing so can be developed. Asteel-ruled die can be used to punch the designated and properlyoriented facet-shaped film pieces, such as for example 1104A and 1104Bin FIGS. 120A and 120B, from the flat film stock 1108 with the preferredorientation 1109, as illustrated. The pre-cut film stock 1108 can thenbe fed, for example, by an automated die set that simultaneously loadsone section per facet, and applies the necessary conformal pressure(and/or heat) adequate to deform of the film elements 1104 and set thepressure sensitive adhesive layer pre-laminated to the initially flatfilm material 1108. Alternatively, pressure sensitive adhesive can bepre-applied to the faceted substrate, as can numerous other adhesivebonding agents, such as uv curing epoxy. Other than the radial facets1102 shown in FIG. 120A, and previously in FIGS. 48 and 49, similarresults can be obtained using other segmented transformation geometries,but the deeper the conicoidal curve, the more segments are used to matchthe film section to the preferred orientation. With precisely cut filmpieces 1104, the registration of adjacent film pieces at the facetboundaries will permit use with any of the above polarization selectiveforms of the optical system 10 since there will be enough mixing withinthe output beam that any slight optical discontinuities at the facetboundaries will not be carried through to the projection screen 26.

In FIG. 121 is illustrated another two-reflector polarization recyclingembodiment for efficiently pre-polarizing the un-polarized lightgenerated, for example, by the light sources 808 and 897 of FIGS. 92 and88, respectively. This embodiment is shown in a longitudinalcross-section in FIG. 121A with the ellipsoidal light source 808 of FIG.92, and in FIG. 122 for the paraboloidal light source 897 of FIG. 88.The embodiment uses a special variation on the form of the split-imageoptical system 10 of FIG. 13. The elements in FIG. 121 are circularlysymmetric about the optic axis 100 and un-polarized light beam 1118converging towards focal point 822 is split into two still converging,but orthogonally polarized light beams 1121 and 1119. The firstpolarized beam 1121 continues along the original direction towards thefocal point 822, but the second polarized beam 1119 is folded by acircularly symmetric conicoidal mirror 1116 along a different path(a-b-c as opposed to a-c), but ultimately to the same focal point 822.Polarized rays are redirected towards the conicoidal mirror 1116 by atransparent 45 degree conic refractive element 1120 made of plastic orglass and fitted with a polarization selective reflecting surface layer1122, preferably the wide band 3M dielectric multi-layer stack filmdiscussed hereinbefore, which for example passes P1 and reflects P2. Thecircularly-symmetric and converging, ray bundle 1118 exits theellipsoidal reflector 820 heading towards the reflector's focal point822, and then encounters the refractive element 1120 on the way. Thissubstrate of the conic refractive element 1120 can be made of eitherglass or plastic. In order to assure optimal alignment of the axis ofsplitting layer 1122 with the out-going polarized beam 1112, the conicrefractive element 1120 is faceted in the manner described, for example,in FIGS. 120A and 120B. Ray bundle 1118 impinging on the conicrefractive element 1120, splits equally into two orthogonally polarizedgroups of rays, one group that passes straight through the conic element1120 towards the focal point 822, and another group that is re-directedradially towards a new radial focal point 1126. The focal point 1126 isactual the folded location of the focal point 822. Consider for examplethe illustrative ray paths a-b-c and a-c. Ray 1128 is emitted by the arcsource 833 and is re-directed by the ellipsoidal reflector 820 towardsthe focal point 822, as the ray 1124. This ray 1124 is then split by theselective reflecting surface layer 1122 into the transmitted ray 1112and the re-directed ray 1114. The re-directed ray 1114, heading for thevirtual focal point 1126, impinges on the shaped reflecting rim of theconicoidal mirror 1116, which can be integrally constructed or added asan extension on the ellipsoid reflector 820. Alternatively, this toricreflecting surface of the conicoidal mirror 1116 can be made as part ofthe conic refractive element 1120. The conicoidal mirror 1116 iscomposed of the same two-layer polarization re-directing and convertingstructure introduced above in numerous examples such as the reflectorelement 1022 in FIG. 114. The re-directed ray 1114 is reflected at thesurface of the conic element 1120 because its polarization P2 isorthogonal to the polarization P1 that is highly transmitted by themulti-layer selective reflecting surface layer 1122. When there-directed ray 1114 strikes the conicoidal mirror element 1116, it isredirected as output ray 1132 in a polarization state that can be madeeither P1 or P2. Whether the output ray 1132 is of polarization P1 or P2depends on the composition of the mirror element 1116. If the mirrorelement 1116 does not contain a quarter-wave conversion layer, theoutput ray 1132 will be of polarization P2. If the element 1116 containsa quarter-wave conversion layer 1119, as in the embodiments of FIG. 114,the output ray 1132 will be of polarization P1. Hence, the output raybundle 1134, as shown in the beam cross-section of FIG. 121B, has acircular cross-section containing an inner core 1136 of polarization P1corresponding to the ellipsoidal light source's original beam diameter,and an annulus region 1138 containing the recycled ray flux, whosepolarization is arranged as either P1 or P2. Making the upper half ofthe beam polarized as P1, and the lower half beam polarized as P2,however, is also possible, and is accomplished by using one set ofpolarization selective reflecting materials for the upper portion of theconic element 1122 and an orthogonally-polarizing set for the lowerportion of the conic element 1122. For example, as shown in FIG. 121C, apolarization selective reflecting layer 1122U that passes P1 andreflects P2 is applied to only the upper half of the conic element 1120,and, a polarization selective reflecting layer 1122L that passes P2 andreflects P1 is applied to only the lower half. In this manner, the raystransmitted through the upper half of the conic element 1120 will be inpolarization state P1, and those transmitted through the lower half ofthe conic element 1120 will be in polarization state P2. Thus, all raysreflected towards the upper half of the mirror element 1116 by the upperhalf of the conic element 1120 and its selective reflecting layer 1122U,will be converted to P1, and become part of the upper half of the outputbeam 1134. All rays reflected towards the lower half of the mirrorelement 1116 by the lower half of the conic element 1120 and itsselective reflecting layer 1122L, will be converted to P2, and becomepart of the lower half of the output beam 1134. This approach waspreviously used in the embodiments of FIGS. 114 and 115.

Since the re-directing surface in the embodiment of FIG. 121 has aconstant slope, the rays originally heading to a focus at the point 822,instead are directed towards a locus of focal points on the ringsurrounding the system's optic axis 100 of radius equal to the distancebetween the optic axis 100 and the focal point 1126. In the embodimentillustrated in FIG. 121A, the toric mirror element 1116 is preferablyhyperboloidally-shaped, with one focus at the (virtual) point 1126 andthe other at the point 822.

In another embodiment illustrated in FIG. 122, the double mirrorarrangement can be fed with collimated rather than converging inputlight, either by using the paraboloidal light source 897 of FIG. 88 orby inserting a negative lens 1140 at the output of the ellipsoidal lightsource 808 of FIG. 92, as illustrated. When the negative lens 1140 isused at the input to provide collimated light, and a positive lens 1143is used at the output to re-converge the collimated light to point 822,as in FIG. 122, the mirror element 1116 of FIG. 121A becomes a 45 degreeplane conic section. The same result can be obtained without thepositive output lens when the mirror element 1116 is formed as anoff-axis toric paraboloid. This method can, for example, be applied, inthe manner of FIG. 121, to form an output beam of a single polarization,one with one polarization state in the beam's inner core 1136, and itsorthogonal state in the annulus 1138, or one with one polarization statein the upper half of the beam and its orthogonal state in the lower halfof the beam. It is this latter configuration where the beam isbifurcated into two orthogonal polarization states that is preferablefor use with the split-image optical system 10.

In the embodiment of FIGS. 122 and 123 the ellipsoidal light source 808of FIG. 92 is combined with a negative lens 1140 to provide collimatedlight 1142 to conic element 1144 made with polarization (selectivereflecting) splitting layer 1122 and re-directing/converting layers 1148and 1150 of the axially-aligned toric mirror 1116′. In the embodiment ofFIG. 123A, two additional axially-aligned mirrors are added, asdiscussed previously, to provide a means for beam shape transformation.An axially aligned concave mirror 1152 of the previously described twomirror beam shape recycling mirror-set of for example FIGS. 95 and 96 isplaced on the output surface of the conic element 1144 and hidden withininterior occluded region 832 of the input beam 1142. The reciprocatingtoric mirror 1158 of the two mirror beam shape recycling mirror-set isformed on the interior surface of conic beam displacer (or expander)1156. The concave mirror 1152 (which can also be convex, as discussedearlier re FIGS. 90, 91, 93, 94, 96, 102 and 103) and second concavemirror 1158, share a common focal point 1160 and, for the presentcollimated light embodiment, each are parabolically shaped (or modifiedparabolically shaped) in profile. Moreover the uniformity enhancingde-focusing adjustments discussed earlier involving aspherizing termsand multiple focal point positions are used in this embodiment as well.Illustrative source ray 1162 leaves the arc source 833 at the point 1130and is re-directed by the ellipsoidal reflector 820 as ray 1164. Thisray 1164 is refracted by the negative lens 1140 such that it emerges assubstantially collimated ray 1166 on the output surface of the negativelens 1140 and proceeds, left-to-right through the conic element 1144until it strikes the beam-splitting surface layer 1122, which as above,divides the collimated ray 1166 into two rays, 1170 traveling upwards inpolarization state P2, and 1172 proceeding left-to-right as beforeparallel to the optic axis 100 in polarization state P1. The ray 1172proceeds generally left-to-right unimpeded until it is displaced outwardalong its path 1174 through the conic beam displacer 1156, and becomes apart of the polarized output bundle as output ray 1176. The upwardorthogonally-polarized ray 1170 in polarization state P2 is re-directedto the right by the toric mirror 1116′ and the action of itsre-directing and converting layers 1150 and 1148, as previouslydescribed, and becomes ray 1178 in polarization state P1. The beamcross-section at line B—B in FIG. 123A just before recycling concavemirror 1158 is shown in FIG. 123B. Outer beam diameter 1180 (see FIG.123B) corresponds to the beam enlargement due to annulus 1182 ofrecycled polarization P1. Interior beam diameter 1184 corresponds tooriginal beam diameter 1186 of the ellipsoidal light source 808 (seeFIG. 123A) enlarged slightly by the collimating action of the negativelens 1140. Dotted diameter 1188 in FIG. 123B corresponds to thecylindrical layer location of the ray 1178 (also shown as a pointlocation in FIG. 123B). The ray 1178 exists outside the rectangularbeam-shape 1192 in FIG. 123B that is the preferred output. Accordingly,the ray 1178 strikes the concave mirror (shaded) 1158 at its uppermidpoint and is re-directed (or recycled) back through focal point 1160and the mirror element 1152. Other features of interest in FIG. 123B arethe inner most diameter 1190, which corresponds to the diameter of thereciprocating mirror element 1152, and also the diameter of the inputbeam's occluded region 832 (enlarged slightly by the negative lens1140). All the so-recycled peripheral rays, that is all rays passingleft-to-right that fall in between the mirror's rectangular opening 1192and the beam's outer diameter 1180, are returned as output rayssubstantially within the interior region diameter 1190. After strikingthe mirror element 1158, the ray 1178 is re-directed downwards throughthe focal point 1160, to the mirror element 1152, whereupon it isre-directed once again as a substantially collimated ray travelingleft-to-right towards the conic beam displacer 1156. Ray 1178 istraveling in the cross-sectional view of FIG. 123A, and as such hits themirror element 1158. Had the ray 1178 been traveling in a some othercross-sectional slice, such as for example a diagonal slice 1194 shownin FIG. 123B, instead of central slice 1196, the ray 1178 would havemissed being clipped by the mirror element 1158, as illustrated by thepoint 1178′ in FIG. 123B. If this were the case, ray 1178 would havepassed through the mirror's rectangular opening 1192 as an output raysubject only to the beam displacement of the conic beam displacer 1156.The mirror element 1152 used in this example collimates all incomingrays, such as the ray 1178, which passes through (or very near) thefocal point 1160. The so-polarized and rectangularly-shaped output beamcross-section is shown in FIG. 123C. Inner diameter 1198 corresponds tothe light ray bundle that proceeded from the arc source 833 as describedabove, but that passes through the conic element 1144 and itspolarization selective reflecting layer 1122. This bundle is bounded, inFIG. 123A and 123B by ray paths 1202 and 1204. Innermost diameter 1206in FIG. 123C is the expansion of the interior diameter 1190 of FIG. 123Bdue to the action of the conic beam displacer 1156. Rectangular aperture1208 corresponds to the outermost boundary of the output regioncontaining rays, and thus represents the transformed beam's outputprofile. This rectangular aperture 1208 is inscribed within the circularregion of diameter 1210 which corresponds to the natural outputcross-section of the ellipsoidal light source 808, in the absence of thereciprocating mirror elements 1152 and 1158. The central or axial pointin each of FIG. 123B and 123C corresponds to the optic axis 100(equivalently, the system 10 axis of symmetry).

It is also possible to produce a rectangularly-shaped polarized outputbeam compatible with the projection systems 10 by means of the beamshape and angle transforming system described in FIGS. 99A-99C. To dothis, the same approach is used as described above, with orthogonallyoriented polarization selective reflecting layers 901 and 901′ appliedto the upper and lower halves of the associated light beam (see FIG.99C). If these selective reflecting layers 901 and 901′ are applied tothe plane surface indicated by 903 on the angle transformer 902 in FIG.99A, the output light 910 will be polarized, for example, as P1, but theorthogonal half with the polarized light flux, P2, will be turned backinto the transformer 902, heading generally right-to-left on its wayback through this transformer 902, and its input aperture 908, by totalinternal reflection at its dielectric boundary side-walls 1212, to theellipsoidal or modified ellipsoidal reflector 820 and the arc source833. If, however, the above polarization selective reflecting layers 901and 901′ are applied to a faceted, conic or curved surface, such as theexample of faceted surfaces 885A-885D in FIG. 99C, substantially all thereflected light flux polarized as P2 can be arranged to remain withinthe element 902 by total internal reflection at its dielectric boundaryside-walls 1212. Therefore, reflections which reverse the direction ofray travel from substantially right-to-left to substantiallyleft-to-right, cause substantially all the once rejected rays tore-appear at the rejecting surfaces 885A-885D and their selectivereflecting layers 901 and 901′, with practically no rays lost by theirpassing left-to-right back through the aperture 908 (see FIG. 99B).These recycled rays of polarization P2 continue to recycle in thismanner until they convert to polarization P1. Any rays arriving at thefaceted surfaces 885A-885D in polarization state P1, pass through aspart of the output rays 910. Some polarization conversion can occurduring the multiple total internal reflections at dielectric element902's sidewalls increasing the output light flux proportionally; otherconversions can occur as a result of small amounts of birefringence inthe dielectric medium of the dielectric element 902. For highestpolarization conversion efficiency, however, it is preferable to add awide band quarter wave retardation film layer 899 and 899′ as describednumerous times above, in this instance, just beneath (or to the left of)the polarization selective reflecting layers 901 and 901′ applied tosurfaces 885A-885D. In this manner, the reflected rays of polarizationP2 pass once through this quarter wave polarization converting layer 899or 899′ when first traveling back right-to-left upon rejection at thelayers 901 or 901′, and a second time when returning left-to-righttowards the layers 901 or 901′, thereby converting from P2 to P1 in theprocess.

There is another improvement with regard to the efficiency of theparaboloidal and ellipsoidal light sources 897 and 808 of, for example,FIGS. 88 and 92 themselves. The conventional reflector shapes do nottake into account the finite size of the radiating source, such as thearc discharge indicated as the region 837 in FIGS. 89A and 89B, nor theneed for a bundle of rays of finite extent which will enter the pupil ofa projection lens with an f/# in the region of f/2.5. In particular,neither reflector shape was intended for use with extended sources suchas even the new miniaturized short-arc sources represented in FIG. 89.The smallest arc sources available emit radiation from arc volumesroughly 1.2 mm in cross-section. Both the standard paraboloidal andellipsoidal reflectors such as 848 in FIG. 88 and 820 in FIG. 92 arehighly aberrated for rays (such as ray 1224B in FIG. 124 for example)that are emitted from points, such as the point 130, that are removedfrom their mathematical focal point 1214. The effect of theseaberrations is to cause a significant number of rays emitted from thearc source 833 and reflected at the reflecting surface of the standardparaboloid 848 or ellipsoid 820 in FIGS. 88 and 92 respectively, todeviate from the directions, such as 1220 and 1218, that otherwise wouldtake them through the SLM 14 and subsequently through the pupil 1216 ofthe projection lens 20. The smaller the size of the SLM 14 relative tothe scale of the reflector, the more mis-directed rays from the lightsource 897 or 808 will fail to make the proper passage through theoptical system 10. Similarly, tighter constraints on the projection lens20 reduce the diameter of the lens pupil 1216 and also result in a lossof mis-directed rays. Given the recent practical trend towards the useof smaller and smaller SLM 14 apertures (10 mm by 14 mm) and the rathernarrow angular constraints of rays in their passage through the SLM 14(±10 degrees for the DMD and usually less for the LCD whose contrastratio drops when high-angle light is used), the inefficiency of thesestandard designs is not surprising. It is not uncommon for less than1000 lumens from a 6000 lumen source to effect a passage through the SLM14 and the lens pupil 1216 to the projection screen 26, as was discussedearlier.

One way to minimize the effects of such aberrations is to increase thesize of the example reflectors 848 and 820 relative to the size of thelight source's emitting volume as illustrated in FIGS. 89A and 89B, andto reduce the angular spread of the rays that will ultimately go throughthe SLM 14 and the lens pupil 1216. While these approaches aretechnically feasible, either alone or in combination, they may not bepractical because of system constraints on projection systems 10 such asthe invention disclosed in FIG. 1A where compactness is both animportant technical and marketing differentiator.

Rather than use only traditional paraboloidal and ellipsoidal reflectorshapes, a generalized conicoidal reflector can be used whose shape isdetermined by an iterative process that takes into account the system 10constraints. By generalized conicoidal reflector, or simply conicoidalreflector, we mean multi-dimension, particularly a three-dimensionalsurface function, that while based on a standard ellipsoid, paraboloid,hyperboloid or spheroid, departs from these standard functions by meansof the addition of aspherizing terms, such as a, b, c and d, referred tothe conic equation described hereinbefore as well as below, and set bythe aforementioned iterative process. Since the two most criticaloptical constraints determining the system efficiency apply sequentiallyto the projection lens 20, its entrance aperture 1216 and to theaperture of the SLM 14, the design program is carried out, not bylaunching rays from the arc source 833, but rather by pre-launching aspecific grid of rays from the lens pupil 1216 backwards towards the arcsource 833, a ray set designed to fill the lens pupil 1216 in arepresentative way, so that each ray represents an equal area of thepupil (and fraction of the available flux). Sets of such rays arepre-launched so that all the rays in each set go through one of a smallnumber of specific test points in the SLM 14, whereupon they arelaunched through the lens and reflector system to a target area.Typically four or five points in the SLM 14 are used, namely at thecenter of the SLM 14, at 0.5 of the semi-diagonal, at 0.70 of thesemi-diagonal and at the full semi-diagonal of the SLM 14. This methodis shown in FIG. 125 for the converging conicoidal reflector 1230, theSLM 14, the projection lens pupil 1216, two illustrative grid points1234 and 1236, and a target zone 1238 located near the conicoid's focus1240. This target zone 1238 typically corresponds to the spatial andangular cross-section of the arc source plasma shown in FIGS. 89A and89B, and lies generally in the vicinity of the reflector's focus. Therays are traced in reverse from their launching points on the grid,through the SLM 14, to the surface of the conicoid reflector 1230 andinto the target area 1238. The number of rays which traverse one of thespecified points in the SLM 14 and fall within a designated target areais a measure of the brightness with which that SLM 14 point will appearon the projection screen 26 as in FIG. 1A (optionally weighted by thelamp's actual brightness distribution function as discussed below). Thisformalism determines those constructional parameters which result in themaximum number of rays for each SLM 14 point reaching the target area.In order to secure this result, additional design parameters areintroduced, over and above those implied by the traditional paraboloidalor ellipsoidal shapes. Both paraboloidal and ellipsoidal shapes can berepresented by the mathematical formula:$Z = \frac{\rho \quad H^{2}}{\left( {1 + q} \right)}$

where Z is the distance along the reflector axis of a point on thereflector, ρ is the vertex point, q²=1−(k+1)ρ²H², H (H²=x²+y²) is thedistance of that point 1232 from the axis of the reflector 1230, and kis the conic constant as before. A mathematical representation of themodified paraboloidal or ellipsoidal shape is created by addingso-called and above mentioned aspherizing terms, such as those shown asa, b, c and d:$Z = {\frac{\rho \quad H^{2}}{\left( {1 + q} \right)} + {aH}^{4} + {bH}^{6} + {cH}^{8} + {dH}^{10}}$

The “aspherising terms” enable the “shaping” of the conicoidal reflectorsurface to develop the optimum design, which can be executed either as asmoothly varying surface function or as a Fresnelized surface. In themicrofiche, Appendix 3 (DOIC2), has been developed to enable that thisdesign sequence can be carried out effectively, although any one of thecommercially-available non-sequential raytracing programs, such as forexample, ASAP, Super Oslo, OptiCad or Code V can be programmed for thesame purpose.

The starting point of the program of Appendix 3 is (1) the diameter ofthe lens pupil 1216 and its position relative to the system origin, (2)the diagonal size of the SLM 14, (3) the needed clearance between theplane of the SLM 14 and the closest approach of the reflector 1230, (4)the arc size or a target area as described above, and (5) the angulardistribution of the light emanating from the arc source.

With such input, the program evaluates the parameters of the conicoid1230, and then executes the reverse raytrace on a grid of nominally 1600launching points for sets of rays. Typically four (or five) sets aretraced for points in the plane of the SLM 14. Conformance tests areperformed on these rays as they pass through the system. The firstmeasure of conformance is whether or not a launching point lies withinthe lens pupil 1216, which is circular. This effectively reduces themaximum number of rays in the rectangular grid which might reach thetarget area to (400)(π) or 1256 rays. The second measure of conformancedetermines whether or not when a ray is directed from the reflector 1230to the target area 1238 it lies within the light emitting angle of thelight source (see illustrative angle θ in FIG. 89A). Only those raysthat satisfy this criterion are candidates for acceptance as imageproducing rays. The final test of conformance is to determine that whena ray arrives at an intersection point with a plane through thereflector 1230 axis, it does so within the bounds of the target area.Only rays which satisfy this last criterion are counted as image formingrays.

A measure of the projection screen 26 illumination efficiency is arrivedat by the ratio of grid rays that survive all three conformance criteriato those that survive only the first criterion. Such ratios alsocharacterize the uniformity of projection screen 26 illumination. Whenthe light source used is known to have an angular and spatial variation,such as that shown characteristically in FIG. 89B (for near-fieldspatial variations; far-field patterns, not shown, relate intensityversus angle), these data are arranged in the form of look-up tables,and used to weight the otherwise conforming rays, so as to discounttheir contribution to efficiency accordingly.

In the event that the uniformity of projection screen 26 illumination isnot satisfactory, one method of uniformity optimization involves movingthe arc or target zone center away from the mathematical focal point ofthe conicoid. This adjustment is allowed by the program of Appendix 3.

Each of the aspherizing terms described previously are variedindividually and the results of all variations are used in a so-calleddamped least squares program to determine that set of values providingbest results. Least squares programs are routinely used in the practiceof other optical designs where an exact solution to the problem is notpossible because the constraints imposed by system considerationsoutnumber the number of available system parameters.

A variation on this embodiment, as mentioned above, includes anincorporation of the ray-set definitions that realistically mimic theactual, experimentally-determined, near-field (spatial) and far-field(angular) radiant properties of the light source to be used within theaspherized conicoidal system of FIG. 125, as illustrated, for example,by the double-peaked angular distribution previously illustrated for thed.c. arc source 833 of FIG. 89B. In this case, the data of FIG. 89Bshows a double-peaked near-field radiation pattern typical for a d.c.arc discharge. In cases such as this, where the distribution of lightalong the length of the arc is non-uniform, an appropriate weightingfactor (or weighting factors), proportional to the indicated relativenear-field spatial and far-field angular intensities, is used with eachray that encounters the target area. These weighting factors are thentaken into account in performing the above optimization. Anothervariation on this method uses separate sets of weighting factors foreach of the three primary colors, in cases where the arc source 833radiates differently at each wavelength band of the primary colors.

Yet another variation on this embodiment uses a separate set ofweighting factors according to the importance given to the screenbrightness and the ratio of corner-to-center brightness on the screen.As one example, it might be decided that the overall goals of theprojection system 10 design can best be met by accepting a level ofillumination at the corners of the projection screen 26 that is only 60%of the brightness level at the center of the projection screen 26. Thisconstraint can be satisfied by use of the weighting factor methoddescribed above.

As one illustrative example, consider the case where the 200 mm entrancepupil of an f/2.5 projection lens 20 is placed at a distance of 500 mmfrom the ellipsoidal illuminator of FIG. 92 so that the principal raysof the system are substantially parallel to the optic axis 100, aspreferred both for an LCD and for a DMD. The diagonal of the SLM 14aperture used is taken as 18 mm with a clearance of 10 mm between theSLM 14 and the closest point on the prototype illuminator of FIG. 92.The arc source used is taken as radiating light through an angle of plusor minus 60 degrees, and the length of the arc is taken as 1.5 mm, withan arc width of 1.5 mm. In this example, the arc is presumed to radiateuniformly along its length, but with appropriate angular weightingfactors applied to actual experimentally-determined radiant distributiondata, a more realistic result is just as readily obtained. Theconstraints of this system are met if the prototype ellipsoid has aneccentricity of 0.994, with a major semi-axis of 266.2 mm and a minorsemi-axis of 28.77 mm. The center of the arc is located at the firstfocus of this ellipsoid. Under these conditions only 1240 rays out of apossible 1256 pass through a point at the center of the SLM 14 andencounter the target area represented by the arc source within the givenplus or minus 60 degrees of the light emitting angle. Of these rays,however, only 800 pass the final criterion of encountering the targetarea within the bounds of the arc size. This means that the maximumpossible brightness of the image of a point at the center of the SLM 14has not been achieved under the constraints stipulated for theconventional ellipsoidal illuminator of FIGS. 92 and 125. Moreover,performing the same analysis for rays which pass through a point at thecorner of the SLM 14 shows that only 324 rays meet the final criterion.

These results can be improved slightly, at least in the center of thefield, by moving the arc 0.25 mm further from the pole of the ellipsoid.When this adjustment is made, the number of rays though a point at thecenter of the SLM 14 which satisfy all criteria increase from 800 to1052. Yet, at the same time, the number of rays through a point at thecorner of the SLM 14 which satisfy all criteria actually drops from 324to 292. In order to obtain this increase in the number of rays throughthe center of the SLM 14 and at the same time increase the number ofsatisfactory rays through a point at the corner of the SLM 14, we cansee that the simple ellipsoidal surface is inadequate, and that a morecomplex conicoidal surface function is preferred.

As discussed above, the additional adjustable parameters preferred areprovided by the conicoid's aspherizing terms a, b, c, and d. As oneexample of this adjustment, consider the case when the aspherizing term,a, is set at (0. 1)10⁻³. The effect of this perturbation taken, forexample, with the aforementioned 0.25 mm displacement of the arc source1238 from the focus 1214 of the unperturbed ellipsoid as in FIGS. 125and 126A is to decrease the number of axial rays 1236 from 1052 to 1028,but to increase the number of rays at the edge 1233 of the SLM 14aperture from 292 to 324. As yet another example invoking additionalaspherizing terms, consider the case when a is (0.6)10⁻³, b is(0.2)10⁻⁶, c is (0.1)10⁻⁸, and d is set at zero. In this case, thenumber of axial rays 1236 increases 1.18 times (18%), and the number ofrays going through the edge of the SLM 14 aperture increases 1.31 times(31%). In each illustrative example, the increases and decreases in thenumber of rays meeting the criteria listed above are referenced to thecase of the standard, unperturbed, ellipsoid. With a completeoptimization of the conicoidal form for the above constraints, it ispossible to improve system throughput by as much as about 1.5 times(50%) depending on system details. The efficiency improvement, ingeneral depends on the specific set of constraints and dimensionsselected, and the corresponding location of the arc source 1238 centerwith respect to the focus 1214 of the ellipsoid 1230.

Each conicoidal adjustment yields an efficiency increase (or decrease)corresponding to each of the indicated test points 1231 in the SLM 14aperture, as in FIG. 125. When separate weighting factors are used forthe color dependent radiating characteristics of the arc source 1238, asmentioned above, the number of efficiencies, so determined, ismultiplied by three, one set for each of the three primary colors (i.e.red, green and blue). Determining the optimum adjustment, therefore,depends on the set output criteria established by the system designerfor each specific projection system 10 arrangement and market objective.In this manner, the optimization can be applied to achieve a particularcolor balance, uniformly across the projection screen 26, or it can beapplied to constrain an acceptable range of red, green and bluedifferences, while maximizing the brightness in the center of the screen26. The optimization can also be applied to increase brightness by someamount at every point on the screen 26, or to sacrifice some brightnessincrease in the center of the screen 26, to increase brightness by agreater amount in the corners of the screen 26. Whatever the outputcriteria, the above adjustments can be performed to find the bestpossible conditions for meeting them.

In some cases, it can be preferable to add a substantially telescopiclens pair 1321 to the modified conicoidal system of FIG. 125, as shownin one possible form (a Galilean telescope) in FIG. 126C, using as anexample, a generalized ellipsoidal reflector 1230. It is also possibleto use an inverted telescope form. Adding the lens pair 1321 increasesthe effectiveness of the above optimization method, as will be explainedhereinafter. In the Galilean telescope form, parallel or substantiallyparallel rays of light traveling right-to-left from the SLM 14 firstencounter negative lens 1319, which forms a virtual image at the focalpoint 822, also the focal point of a positive lens 1317. The rays thatemerge right-to-left from the positive lens 1317 do so as collimated orsubstantially collimated. The magnification of the lens pair 1321 isequal to the diameter of the ray bundle emerging right-to-left from thepositive lens 1317 divided by the diameter of the ray bundle enteringthe negative lens 1319. In the case of an inverting telescope, raysconsidered right-to-left as above, the first lens encountered is apositive lens that forms a real image at its focal point, which lies atthe focal point of a larger positive lens further to the left towardsthe reflector 1230. The magnification, in this case, is based on thesame diameter ratio as above. In either case, however, a field-stop canbe inserted at the common focal plane to define the area to be coveredby the field of illumination.

The inclusion of telescopic or approximately telescopic lens systems inthe optical systems 10 reduces the spread of light rays about theprincipal rays and thereby increases the number of rays generated by thelight source 12 that participate in the projected image on theprojection screen 26. As discussed above, the light sources 12 based onthe standard paraboloidal or ellipsoidal systems of FIGS. 88 and 92 showconsiderable aberrations, mainly in the form of higher order coma andoblique spherical aberration. Although these aberrations are controlledto some degree by the aspherizing methods described above, furtherimprovement is still possible. One means for extending the range towhich such aberrations can be alleviated is by adding the approximatetelescopic lens pair 1321 as shown in FIG. 126B, comprising the positivelens 1317 and the negative lens 1319 (or two positive lenses aspreviously described). Moreover, aspheric surfaces can be added on oneor both such lenses to further increase the degree to which aberrationscan be reduced and/or to provide an independent means of light controlbeyond that of only the modified conicoidal surface described above. Bymeans of this type of lens pair, the spread of rays about the principlerays is reduced, as in a previous example, from plus or minus 11 degreesto a value of plus or minus (11)/M degrees, where M is the magnificationof the approximately telescopic system. The form of the aspherized(ellipsoidal) conicoid is such as to bring the principal rays to anfocus at the appropriate focus of the conicoid. Accordingly, the samereduced spread of the rays surrounding the principal rays results inthis conicoidal case, in a reduced aberrational spread of the rayssurrounding the principal rays. This in turn translates into the abilityto make more rays satisfy the above system constraints, which therebyincreases the effective system efficiency beyond the level possible byaspherizing the conicoidal reflector of FIGS. 125 or 126A by itself.

One variation on this telescopic method, is to apply the aspherizingterms on the telescopic elements themselves (or alternately, on anyother lens elements or plates in the system 10), for example, to controlthe light emanating from just one portion of the peaked light sourcedistribution shown in FIG. 89B, while letting the separate second set ofaspherizing terms on the conicoidal reflector surface apply to the lightemanating from the other portion of the peaked light sourcedistribution. Once again, the final surfaces can be either smoothlyvarying conicoidal functions or they can be fresnelized. This approachmakes it possible for more of the rays from such a non-uniform lightsource 833 to satisfy the conformance criteria than would be the casewere all aspherizing terms applied with respect to an average pointchosen in the center of the arc source of FIG. 89B or with respect toone of the two peaks and not the other.

The use of two or more sets of spherized conicoidal surfaces asdescribed hereinabove, can also be applied to achieve more independentcontrol of the number of effective axial rays versus the number ofeffective rays at the edge of the SLM 14 aperture. When only one surfaceis aspherized, such as that of the conicoid reflector 1230 of FIG. 125,adjustments that increase the number of effective axial rays cancorrespondingly decrease the number of effective rays at the edge of theSLM 14 aperture, or visa versa. Using two different aspherized surfaces,however, allows the aspherizing terms applied to one aspherized surfaceto optimize, for example, the number of effective axial rays, while theaspherizing terms applied to the second aspherized surface can optimize,for example, the number of effective rays at the edge of the SLM 14aperture. In this case, the best location for the two aspherizedsurfaces is that which causes the maximum possible independence betweenthe two simultaneous optimizations.

The method of FIG. 125 as described above and as executed with, forexample, the program given in Appendix 3, is applicable to the design ofa continuous, integral piece for the conicoid reflector 1230 as shown inFIG. 125. It is also applicable, by extension to the more complicatedseries of multiply ogived or connected toric conicoid sections shown inFIGS. 126A and 126B. Since the emission of most of the arc sources 833is generally circularly symmetric (or nearly so) about the arc source'selectrode axis, whenever that electrode axis is aligned with theprojection system's optic axis 100, the reflector used to redirect thearc's emission is preferably made circularly symmetric as well, unlessthe method of FIG. 125 is otherwise applied to transform the lightsource's output beam cross-section to a non-circular format.

The most common conventional method for achieving color images using theLCD 14 is to incorporate three identical LCD's, one for each primarycolor: red (R), green (G) and blue (B). Color selective (dichroic)filter materials are ordinarily used for this purpose in conjunctionwith conventional mirror elements that spatially separate the whiteinput light into the three color bands, and pass these separate colorsthrough respectively separate LCDs. The three resulting mono-coloredimage beams are re-combined into one, and projected onto the viewingscreen with perfect pixel-to-pixel registration. The most compact of theconventional methods uses a prismatic cube 1246 with dichroic filterlayers on the internal prism faces, as shown in FIG. 127.

Preferred embodiments of the instant invention which operate with thesplit-image optical systems 10, are given, for example, in FIGS.128-130. In the system of FIG. 128, unpolarized light 1248 is suppliedby one of the four CURL sources 916, 918, 920 and 922 of FIGS. 105 and106. The rectangularly-shaped narrow-angle beam 1248 enters thefour-prism (1249, 1250, 1251, and 1253) polarization beam splitter 23and proceeds upwards. A first beam-splitting layer 1252 reflects P2 andpasses P1. The nomenclature WP1 and WP2 designates “white” P1 and“white” P2 respectively, with the same designation applied to R, G, andB as well.) A second polarization beam-splitting layer 1254 is orientedto pass P2 and reflect P1. Intermediate layers 1256 and 1258 arelaminated to each other with the layer 1258 above the layer 1256. Thelayer 1256 is a wide band half wave polarization converting film thatconverts WP1 into WP2. The layer 1258 is preferably a high-transparencyabsorption-type polarizer aligned to absorb any residual P1 afterconversion by the layer 1256. Boundary layers 1262 and 1260 are awide-band quarter-wave polarization converting film and a metal ormetallic reflecting film, respectively, as described numerous timesabove. Their purpose, as before, is to reverse both the incident light'spolarization and direction. All the four prisms 1249, 1250, 1251, and1253 are preferably are Porro prisms. Adjacent prism elements 1264 and1266 of splitter section 22 re-direct the output beams from the upperand lower regions 82 and 84 of the respective LCD 14 (R, G, and B)images and elements 1268 and 1270 cause the light to point at preciselythe oblique angles preferred by the projection system 10 mirrors. Exitaperture layers 1272 and 1274 remove substantially any traces of thewrong polarization from the beams. In this case, the upper (preferablytelecentric) projection lens 1276T projects polarization P2, and theaperture layer 1272 is arranged to pass P2 and absorb P1.

The color and polarization separations are illustrated in FIG. 128 forthe unpolarized light (white) 1248. The solid path shows how leftwardheading WP2 is filtered into RP2, BGP2 and then BP2 and GP2. The solidray path also details how RP2 travels through the upper half of the redLCD 14RL, reflects and changes polarization and re-traces its path asRP1, eventually entering lower projection lens 1276L as RP1. The dottedpath shows similar details for the upward travel of the WP1 ray, whichis split into the primary colors, all of which enter the upperprojection lens 1276T as RP2, GP2 and BP2, representing imageinformation from the upper image region 82 of the LCD 14.

In the arrangement of FIG. 128, it is assumed that the upper and lowerimage regions 82 and 84 of the LCD 14 correspond to the actual upper andlower portions 86 and 88 of a complete image on the projection screen 26(see FIG. 1A for example). It is also possible for special viewingembodiments that each regions is programmed electronically to bedifferent views of the same image (e.g., left eye and right eye) withspecial adaptations of the methods optical systems 10 (as will beintroduced below) or more conventional folded-optic systems arranged tosuperimpose these two images on each other in a way that produces athree-dimensional image when viewed with proper polarizing glasses. Thisembodiment will be discussed in more detail hereinbelow.

A variation on FIG. 128 is shown in FIG. 129 and is suitable for thesplit-image optical systems 10 using a single image beam, such as, forexample, in the inventions of FIGS. 14-20, 32-38 and 54. In thisembodiment, the output beam-splitter 22 and corresponding projectionlenses 1276 of FIG. 128 are replaced by the single telecentricprojection lens 1276. The R, G, B image information from the top orupper region 82 of the LCD 14 is retained in polarization state P2, andthe image information from the lower region 84 of the LCD 14 is retainedin the orthogonal state P1. These two polarization states can be used,as mentioned above, to facilitate three-dimensional viewing, each colorimage being in an orthogonal polarization, or the two polarizations canbe separated post-projection of the lens 1276 by an output beam-splitter22, such those illustrated previously in FIGS. 79 and 81-83 used inconjunction with the split-image portions of a single beam full-screenimage, as with any of the split-image projection system 10 embodiments.

In another embodiment given in FIG. 130, the output light provides thecolor image in one polarization state, P1. This format is appropriatefor the projection system 10 methods of, for example, FIGS. 14-20, 32-38and 54, where an image separator or the buffer zone 148R, 148B and 148Gis needed, but where the image information is preferably in a singlepolarization state. In this case, the half-wave polarization convertingelement 1256 of FIGS. 128-129 is eliminated and the polarizationfiltration element 1258 used above to remove unwanted P1 is replacedwith element 1259 to remove unwanted P2.

A further variation of the embodiment of FIG. 129 is given in FIG. 131.In this case, the rightward output from the projection lens 1276 exitaperture is separated into two orthogonally polarized beams bybeam-splitter 22 and the method of FIG. 81, the aperture layer 1272acting to purify the output polarization P2, and the aperture layer 1274purifying the output polarization P1, both from residual traces of theirorthogonal polarization states. This embodiment is suited to use withany of the split-image projection system 10 methods, and can also beadapted for three-dimensional viewing.

Yet another variation on the embodiment of FIG. 128 is given in FIG.132, in this case with an alternative system 23 for processing lightfrom one of the four collimated (optionally rectangular cross-section)light (CURL) sources 916, 918, 920 and 922 of FIGS. 100-103. In thisinstance, a polarization separator and coupler 23 is used basedgenerally on the methods of FIGS. 104 and 105, and is positioned betweenthe standard color splitting cube shown in FIG. 78 comprising the threeLCDs (or SLMs) 14R, 14G, and 14B and the simple polarizationbeam-splitter 22 of FIG. 128. This method also eliminates the half-wavepolarization converting element 1256 of FIG. 128 and uses the purifyingelement 1259 to removes any traces of P2.

Still another variation on the embodiment of FIG. 128 is given in FIG.133. This embodiment employs a two-stage polarization processor 1280 thesecond stage of which provides means for coupling polarized lightbetween the color splitting cube 1247 and its three LCDs (or SLMs) 14R,14G and 14B and the polarization beam-splitter 22. The prism elementscomprising the first stage of the polarization processor 1280, outputwhite light in two equally polarized beams, one in polarization P1 andthe other in the orthogonal polarization state P2. In this case, theleft-hand side 3M-type polarization selective reflecting film layer 1254transmits WP2 (“white” P2 as above) and reflects WP1 to the right, theorthogonal polarization from the unpolarized incident light 1278originating on the left-hand side of the chosen CURL source 916, 918,920 or 922. This reflected light is sequentially converted to WP2 by theaction of half-wave converting layer 1284 and then filtered to removeany trace P1 by the action of the sequential filtration element 1258,preferably a high-transmissivity absorption-type polarizer, aspreviously discussed. This filtration step assures that WP2 is purifiedwith regard to any contaminating WP1, which, as has already beendiscussed, is critical to the methods of projection system 10. Theconverted WP2 proceeds to the right until it is sequentially processedby the converting and reflecting boundary layers 1260 and 1262, whichact to reverse both polarization state and direction, so that WP1 isout-coupled by reflection at the polarization selective beam splittinglayer 1252. Unpolarized light from the right-hand side of the CURLsource 916, 918, 920 or 922 used is handled in a similar manner.

The internal light within the processor element 1280 is therebypolarized in two beams, both proceeding right-to-left into the LCDcolor-splitting prism coupling cube 1247. The two beams are firstprocessed within the processor 1280, by a bi-directional prism-couplingcube formed by two Porro prism elements 1288 and 1290, and anintervening layer of two orthogonally oriented 3M-type polarizationselective reflecting layers 1252 and 1254, each covering one half of thediagonal interface between the prism elements 1288 and 1290. In thismanner, left-hand side light rays WP2 from the processor 1280 interiorproceed upwards until striking the beam splitting layer 1252, whereuponthey reflect to the left, and head into the aforementioned LCDcolor-splitting cube 1247. In the cube 1247 the light rays are splitinto rays of primary colors R, G and B, passed into and out of theassociated LCDs 14, and reversed in polarization by their round-trippassages through the LCDs 14, recombining on the horizontal axis beamsplitting layer as superimposed rays of R, G, and B in polarizationstate P1. These rays are passed through the polarization selectivereflecting layer 1252, and subsequently split upwards and out to thetelecentric projection lens 1276T by the action of reflective layers1292T and 1292L. These layers can be, for example, identical plane metalor metalized reflectors or polarization selective reflecting layers, and1292T passes P2 and reflects P1, while 1292L is made to pass P1 andreflect P2. The same mechanism applies to light from the right-hand sideof the polarization processor 1280, through the action of the 3M-typepolarization selective reflecting layer 1254, which reflects WP1 andpasses R, G, B rays in polarization state P2.

An alternative variation on the method of FIG. 133 is given in FIG. 134.In this case the CURL sources (one of the 916, 918, 920 and 922) isoriented 90 degrees to the orientation of FIG. 133, requiring the use ofa different polarization processor. The ability to have alternativeorientations of the light source component train is important whenfinding component orientations that lead to the minimum volume for aparticular projection system method and cabinet. In this case, thepolarization processor is arranged for horizontally-oriented input lightand vertically-oriented output light. The processor element 1280 of FIG.133 was arranged for vertically-oriented input light andvertically-oriented output light. In the method of FIG. 133, light fromthe upper region 82 and the lower region 84 of the LCD image were inorthogonal polarization states, separated by the beam-splitter 22, andprojected using the two separate projection lenses 1276T and 1276L. Inthe embodiment of FIG. 134 it is preferable, though not required, toimage this light directly with the telecentric projection lens 1276, andperform the beam-splitting function after (to the right of) theprojection lens 1276, as in the method of FIG. 131.

The methods of FIGS. 128-134 involve one LCD (SLM) 14 for each of thethree primary colors, R, G and B. The LCDs 14 are physically dividedinto upper and lower regions 82 and 84, each region corresponding to onehalf of the complete image to be projected by the methods describedabove. Each region of the LCD 14 is magnified by the optical system 10and applied to the upper and lower portions 86 and 88 of the projectionscreen 26, where the complete magnified image is reconstructed as awhole. In another embodiment the two orthogonally-polarized imageportions could alternatively represent different views or perspectivesof the same image scene and be superimposed on each other in such amanner that three-dimensional viewing were made possible. Suchthree-dimensional viewing using the split-image LCD approachesdescribed, sacrifices image resolution, as each of the LCD image regions82 and 84 must contain a complete image. This means that if the LCD 14were, for example, of 1280×1080 resolution, the three-dimensionalfull-screen projected image would appear as if 640×540 in resolution,provided other electronic means were not applied to compensate for thisdilution.

It is possible to avoid a loss of resolution, however, by using two ofthe LCDs (or the SLMs 14) rather than one for each primary color imageregion. One possible embodiment for doing so is shown in FIG. 135. Inthis embodiment, two identical color splitting LCD 14 prism cubes 1247Aand 1247B each consisting of the three LCDs 14 as above, 14RL, 14GL,14BL and 14RT, 14GT, 14BT, sharing a mutual optic axis 100 are orientedin mirror symmetry to a plane perpendicular to the optic axis 100, andseparated by polarization processing cube 1294, which was usedpreviously as the beam splitter 22 in the embodiment of FIG. 131. Inthis embodiment, the polarization processing cube 1294 ismulti-functional, in that it simultaneously directs input light of onepolarization to the left-side color-splitting LCD 14 prism cube, directsinput light of the orthogonal polarization to the right-sidecolor-splitting LCD 14 prism cube, and it outputs the resulting mixtureof polarized R, G, B light beams produced by each left-side andright-side color-splitting LCD 14 prism cubes. Unpolarized verticallyincident light from one of the CURL sources 916, 918, 920 and 922 istransformed into orthogonally-polarized light that is directed leftwardsas polarization WP1 (white P1) and rightwards as WP2 (white P2) into therespective color-splitting LCD 14 prism cubes 1247A and 1247B. Each ofthe color-splitting LCD 14 prism cubes 1247A and 1247B operates aspreviously described and returns color processed image light in theorthogonal polarization state to that which was first applied. In thiscase, the left color processing cube 1247A is fed with white light ofpolarization P1 and outputs colored image light of polarization stateP2. Conversely, the right color processing cube 1247B is fed with whitelight of polarization P2 and outputs colored image light of polarizationstate P1. The multi-functional polarization processor 1294 outputs asingle vertically directed beam within which the two images (one fromthe left-hand color processing cube 1247A and one from the right-handcolor processing cube 1247B) are precisely superimposed as aspatially-organized mixture of R, G and B rays that are sorted by theirpolarization state. The telecentric projection lens 1276 is able toimage each set of the LCDs 14 on precisely the same optical path length,so that a single projected image can be achieved in sharp focus. Sinceeach image is in an orthogonal polarization state and contains the fullresolution of each LCD, the projected image can be viewed inthree-dimensions, without loss of resolution, if the left and rightimages represent different views or perspectives of the same scene, ascustomarily done in three-dimensional viewing systems, as shown in FIG.136. The image appropriate for the so-called “left-eye” viewing isapplied to the driving circuitry for the left-hand LCDs (or the SLMs14), and the corresponding “right-eye” images are applied to the drivingcircuitry for the right-hand LCDs (or the SLMs 14). The associatedmethods for the electronic programming of LCD images has already beendiscussed earlier.

Another variation on the method of FIG. 136 is given in FIG. 137, wherethe polarization beam-splitter 22 of FIG. 131 is used after theprojection lens to provide one image for the lower image region 86 ofthe split-image systems 10 of, for example, FIGS. 1A and 11-13, andanother for the upper region 82 as, for example, in the embodiments ofFIGS. 128-134, but where each image region is applied to a complete LCD(or the SLM 14) rather than to one-half of an LCD (or the SLM 14). Theadvantage of doing this is that the projected image can be made twicethe resolution of the images formed with the single split-LCDapproaches. The only correction that would be applied is that ananamorphic projection lens system would be used to compress each imagehalf into the correct aspect ratio desired. Without compression there-constructed projected image would be of 4×6 aspect ratio, rather thanthe industry-standard 4×3 U.S. TV aspect ratio. There can beapplications where a 4×6 aspect ratio is desirable, or the anamorphiccorrection can be applied to whatever aspect ratio is set upon.

An alternative embodiment of the method of FIG. 137 is given in FIG.138, where the images can be arranged to be superimposed and projectedby the single-polarization projection methods of FIGS. 14-20, 32-38 and54, avoids the need for an anamorphic system, and limits the resolutionto that of a single LCD (unless some form of interlacing is used tointerleave the image rows). In this case, the output of one side of thepolarization beam splitter 22 is modified with a wide-band half-wavepolarization converting film 1296 located to the left of thepolarization purification exit aperture layer 1272. By thismodification, both the lower and upper image regions 84 and 82 arearranged to be in the same polarization state (P1), and when properlysuperimposed can be projected in perfect registration as a single image.

The embodiment of FIG. 137 can be used, alternatively, as in FIG. 138for three-dimensional viewing, provided the LCDs (or SLMs 14) are drivenwith the appropriate left-eye and right-eye material, and the system 10is selected or adjusted as above for superimposed image alignment.

The two-projection lens embodiments of FIG. 137 are given in FIGS. 140and 141 respectively for double resolution split-image projection andfor normal resolution three-dimensional projection.

In FIG. 142 the embodiment of FIG. 128 is modified for the case wheretwo of the CURL sources 916, 918, 920 and 922, rather than one, are tobe used. One advantage of this approach is the potential for increasedscreen brightness. Despite the fact that two unpolarized light sources12 are used, only a single one of the color splitting LCD (or SLM 14)prism cubes 1247 is needed. The composite output beam containssplit-image information in the same polarization state for use withsingle polarization systems 10, such as those of FIGS. 14-20, 32-38 and54.

In the embodiment of FIG. 143, the resulting output beam 1302 codifiesthe split-image information in orthogonal polarization states asappropriate for the split-image projection system 10 methods of FIGS. 1Aand 11-13. In this case, the polarization beam splitter 22 is used, asbefore, in conjunction with two projection lenses 1276T and 1276L, onefor each of the image regions 82 and 84.

The embodiment of FIG. 144 projects the split-images using a single oneof the projection lenses 20 via the single polarization projectionsystems. Output layer 1298 converts one image half from P2 to P1 tomatch the polarization of the lower image region 84, and identicalpolarization purification filters 1300 are used to prevent anycontamination from the orthogonal polarization.

The embodiment of FIG. 145 retains each image region in its orthogonalpolarization and uses the beam splitting method after the projectionlens 1276 to develop upper and lower image beams from the systems 10requiring orthogonal polarizations.

An embodiment is shown in FIG. 146 for the case where the SLM 14 is areflective digital micromirror device (DMD) 14D. In this case somespecial arrangements are needed to assure compatibility with the tiltingmirror DMD. For example, one of the above CURL sources 916, 918, 920 and922 are combined with one of the previously described and appliedpolarization processing methods (e.g., the polarization processor 1310to output collimated and spatially polarized light. This light isfocused by condensing lens (or lens set) 1304 so that the light passesthrough the color sequencing wheel 1306 using the smallest possibletransmission area. The color sequenced light is re-constituted by lenssub-system 1308 and applied to the DMD aperture so as to pass throughthe projection lens 1276 whenever image light is to be projected ontothe projection screen 26. Whenever no light is to be projected, the DMDmirrors are oriented so that light cannot be transmitted by theprojection lens 26 to the beam splitter 22 shown.

An embodiment is shown in FIG. 147 that is a variation on thesplit-image projection system 10 embodiment of FIG. 13 for use with thethree-dimensional viewing capability of the embodiment of FIG. 141. Inthe embodiment of FIG. 141, output light emanates from two projectionlenses 1276T and 1276L, one providing R, G, and B image content inpolarization state P1, and the other providing different R, G, and Bimage content in polarization state P2. The embodiment of FIG. 147utilizes two projection lenses, the beam-generating sub-system 1297 ofFIG. 141, two orthogonally-polarized image beams 1304 and 1306, acrossed set of 3M-type polarization selective reflecting mirror elements1302 and 1303 (each containing a polarization selective reflecting layerand a transparent supporting substrate as illustrated several timesabove), and a set of shaped polarization converting a redirecting mirrorelements 1308T and 1308L, each composed of two curved sections, 1314Tand 1316T, and 1314L and 1316L. The polarization selective reflectingmirror element 1302 is arranged to pass P1 and reflect P2, whereaspolarization selective reflecting mirror element 1303 is arranged topass P2 and reflect P1. Accordingly, light rays from projection lens1276T pass through mirror element 1302, and are converted (to P2) andredirected (towards mirror elements 1302 and 1303) by contact withmirror element 1308T or the appropriate section of mirror element 1308T,either 1314T or 1316T. The mirror elements 1302 and 1303 fold thevirtual source point 1314 to virtual source points 1314T and 1314L, sothat, for example, the shaped mirror element 1308T redirects light rays1306 over the surface of mirror element 1302 as if the rays actuallyoriginated at source point 1314T, and by folding, at source point 1314.As such, the P2 rays emanating from mirror element 1308T, pass throughmirror element 1303 and strike mirror element 1302, whereupon they areredirected towards the Fresnel lens 110 and the projection screen 26,forming a sharply focused image of polarization P2 covering the entireprojection screen 26. The same process extends to the rays 1304 thatemanate from projection lens 1276L in polarization state P2. Ultimatelythese rays pass through mirror element 1303, are converted to P1, andalso form a sharply focused image covering the entire projection screen26. Hence, there are two sharply focused and overlapping images onprojection screen 26, one in polarization state P1, and the other inpolarization state P2.

The cabinet thickness the results with the illustrative embodiment ofFIG. 147 is approximately D/3 and somewhat greater than the D/4 depthassociated with the method of FIG. 13. Other preferred variations of theembodiment of FIG. 147 include curved (conicoidal) forms of mirrorelements 1302 and 1303.

While preferred embodiments of the invention have been shown anddescribed, it will be clear to those of skill in the art that variouschanges and modifications can be made without departing from theinvention in the broader aspects set forth in the claims hereinafter. Inparticular, the various subcomponent elements and systems describedherein, as well as their optical equivalent, can be used in combinationwith, or when operatively proper substituted for, the other elements andsystems set further herein.

What is claimed is:
 1. An illuminating system, comprising: an outputscreen including a liquid crystal display (LCD), means for generatingpolarized white light; a polarized light source system for illuminatingsaid output screen such that light of a first polarization state fromthe means for generating polarized light is converted to polarized lightof a second polarization state for display on said screen, saidpolarized light source system including: (a) a first set of polarizationselective light processing elements including a conicoidal reflectingelement whose vertex contains a small inlet that receives light from themeans for generating polarized white light and passes a focused highnumerical aperture white light of first circular polarization state andalso whose surface converts the white light of a first circularpolarization state into reflected light of a second circularpolarization state, the second circular polarization state beingorthogonal to that of said first circular polarization state; (b) asecond set of polarization selective light processing elements includinga first element which reflects light of a first linear polarizationstate and also transmits light of a second linear polarization state andfurther including a second optical element for converting light of saidfirst circular polarization state into light of said first linearpolarization state and for converting light of said second circularpolarization state into light of said second linear polarization state,thereby enabling output of light of the second linear polarization stateonto the output screen; and said output screen including at least one ofa Fresnel lens, a diffuser for converting the light of the second linearpolarization state passed through said Fresnel lens into output beams ofspecific angular extent, a polarizer for at least one of absorbing andreflecting the light of said first linear polarization state andtransmitting the light of said second linear polarization state, andsaid LCD screen receiving output light.
 2. The illuminating system asdefined in claim 1 wherein said means for generating light comprises anarc source.
 3. The illuminating system as defined in claim 1 whereinsaid first element comprises a conicoid.
 4. The illuminating system asdefined in claim 1 wherein said diffuser comprises a holographicdiffuser.
 5. The illuminating system as defined in claim 1 wherein saidconicoidal reflecting element comprises a metallic material.
 6. Theilluminating system as defined in claim 1 wherein said conicoidalreflecting element comprises a hyperboloid.
 7. The illuminating systemas defined in claim 6 wherein the conicoidal reflecting element has atruncated shape such that its output aperture is cut to enableilluminating the LCD screen.
 8. An illuminating system comprising: anoutput screen including a liquid crystal display (LCD); means forgenerating circularly polarized white light; a polarized light sourcesystem for illuminating said output screen such that light of a firstcircular polarization state from the means for generating circularlypolarized light is sequentially converted to polarized light of a firstlinear polarization state, a light of first circular polarization state,then to a second circular polarization state, and then to a secondlinear polarization state for illumination on said screen, saidpolarized light source system including: (a) a first set of polarizationselective light processing elements including a conicoidal metallicreflecting element whose vertex contains a small inlet that receivespolarized light from the means for generating circularly polarized lightof the first circular polarization state, said inlet passing a focusedhigh numerical aperture beam of said light of the first circularpolarization state, and said metallic reflecting element having ametallic surface that converts the white light of the first circularpolarization state into reflected light of the second circularpolarization state, the second circular polarization state beingorthogonal to that of said first circular polarization state; (b) asecond set of polarization selective light processing elements includinga first element for converting light of said first circular polarizationstate into light of said first linear polarization state and forconverting light of said first linear polarization state into light ofsaid first circular polarization state, and said first element furtherfor converting light of said second circular polation state into lightof a second linear polarization state, and further including a secondoptical element which reflects light of a first linear polarizationstate and also transmits light of a second linear polarization state,thereby enabling output of light of the second linear polarization stateonto the output screen; and said output screen including at least one ofa Fresnel lens, a diffuser for converting the light of the second linearpolarization state passed through said Fresnel leas into output beams ofspecific angular extent, a polarizer for at least one of absorbing andreflecting the light of said first linear polarization state andtransmitting the light of said second linear polarization state, andsaid LCD screen receiving output light.
 9. An illuminating systemcomprising: an output screen including a liquid crystal display (LCD);means for generating circularly polarized white light; a polarized lightsource system for illuminating said output screen such that light of afirst circular polarization state from the means for generatingpolarized light is sequentially converted to polarized light of a firstlinear polarization state, then to a second circular polarization state,and then to a second linear polarization state, for illumination on saidscreen, said polarized light source system including: (a) a first set ofpolarization selective light processing elements including a conicoidalmetallic reflecting element whose vertex contains a small inlet thatreceives said polarized white light from the means for generating saidpolarized light of a first circular polarization state, said inletpassing a focused high numerical aperture beam of said light of a firstcircular polarization state, and said metallic reflecting element havinga metallic surface that converts the white light of the first circularpolarization state into reflected light of the second circularpolarization state, the second circular polarization state beingorthogonal to that of said first circular polarization state; (b) asecond set of polarization selective light processing elements includinga first element consisting of a broad band quarter-wave phaseretardation layer for converting light of said first circularpolarization state into light of said first linear polarization stateand for converting light of said first linear polarization state intolight of said first circular polarization state, and for convertinglight of said second circular polarization state into light of a secondlinear polarization state, and further including a second opticalelement parallel to and in contact with this said first element,consisting of a broad band reflective polarizer, which reflects light ofa first linear polarization state and also transmits light of the secondlinear polarization state, thereby enabling output of light of thesecond linear polarization state onto the output screen, the firstelement positioned to face the curved metallic surface of said first setof polarization selective light processing elements and with theinterface between said first and second elements positioned at adistance from the conicoidal vertex equal to one half the conicoidalelement's focal length; and said output screen including at least one ofa Fresnel lens, a diffuser for converting the light of the second linearpolarization state passed through said Fresnel lens into output beams ofspecific angular extent, a polarizer for at least one of absorbing andreflecting the light of said first linear polarization state andtransmitting the light of said second linear polarization state, andsaid LCD screen receiving output light.